fix(py_venv): Partially resolve PYTHONHOME (#668)

Use the partially resolved interpreter path (eg. the first partially
resolved path for the interpreter binary which is under `.runfiles/`) as
the basis from which to identify the `pyvenv.cfg` file and thus as the
`sys.prefix`.

This fixes an issue which would allow the interpreter to observe an
un-resolved `sys.prefix` relative to which `_aspect.pth` source imports
would be invalid.

Fixes #666.

### Changes are visible to end-users: yes

- Searched for relevant documentation and updated as needed: yes
- Breaking change (forces users to change their own code or config): no
- Suggested release notes appear below: yes

Fixed a bug which would prevent 1stparty and other relative imports from
working as desired when venvs are consumed outside of Bazel.

### Test plan

- Manual testing; please provide instructions so we can reproduce:

Using `chicagotrading/shorty` at `venv-bug-repro`,

```
❯ git diff | cat
diff --git a/MODULE.bazel b/MODULE.bazel
index 5f63c5f..b8d3dea 100755
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -20,3 +20,8 @@ python = use_extension("@rules_python//python/extensions:python.bzl", "python")
 python.toolchain(
     python_version = "3.12",
 )
+
+local_path_override(
+    module_name = "aspect_rules_py",
+    path = "/Users/arrdem/Documents/work/aspect/rules_py",
+)

❯ bazel run //app:app_bin.venv
INFO: Analyzed target //app:app_bin.venv (1 packages loaded, 876 targets configured).
INFO: Found 1 target...
Target //app:app_bin.venv up-to-date:
  bazel-bin/app/app_bin.venv
INFO: Elapsed time: 1.056s, Critical Path: 0.02s
INFO: 1 process: 577 action cache hit, 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/app/app_bin.venv
final  "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/bin/python"
[aspect] venv root "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv" venv.cfg "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/pyvenv.cfg"
[aspect] PyCfg { root: "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv", cfg: "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/pyvenv.cfg", version_info: "3.12.0", interpreter: Runfiles { rpath: "rules_python++python+python_3_12_aarch64-apple-darwin/bin/python3", repo: "_main" }, user_site: false }
[aspect] "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/bin/python3"
[aspect] Attempting to execute: "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/external/rules_python++python+python_3_12_aarch64-apple-darwin/bin/python3" with argv[0] as "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/bin/python3" and args as ["-B", "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/aspect_rules_py+/py/private/py_venv/link.py"]


Linking: /private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv -> /Users/arrdem/Documents/work/aspect/clients/chicago-trading/shorty/.app+app_bin.venv

Link is up to date!

To configure the virtualenv in your IDE, configure an interpreter with the homedir
    /Users/arrdem/Documents/work/aspect/clients/chicago-trading/shorty/.app+app_bin.venv

    Please note that you may encounter issues if your editor doesn't evaluate
    the `activate` script. If you do please file an issue at
    https://github.com/aspect-build/rules_py/issues/new?template=BUG-REPORT.yaml

To activate the virtualenv in your shell run
    source /Users/arrdem/Documents/work/aspect/clients/chicago-trading/shorty/.app+app_bin.venv/bin/activate

virtualenvwrapper users may further want to
    $ ln -s /Users/arrdem/Documents/work/aspect/clients/chicago-trading/shorty/.app+app_bin.venv $WORKON_HOME/.app+app_bin.venv


❯ source .app+app_bin.venv/bin/activate

❯ python3 -m app
final  "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/bin/python3"
[aspect] venv root "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv" venv.cfg "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/pyvenv.cfg"
[aspect] PyCfg { root: "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv", cfg: "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/pyvenv.cfg", version_info: "3.12.0", interpreter: Runfiles { rpath: "rules_python++python+python_3_12_aarch64-apple-darwin/bin/python3", repo: "_main" }, user_site: false }
[aspect] "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/bin/python3"
[aspect] Attempting to execute: "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/external/rules_python++python+python_3_12_aarch64-apple-darwin/bin/python3" with argv[0] as "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/bin/python3" and args as ["-m", "app"]
hi from foo_lib
Hi from __main__

❯ python3 -P -m app
final  "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/bin/python3"
[aspect] venv root "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv" venv.cfg "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/pyvenv.cfg"
[aspect] PyCfg { root: "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv", cfg: "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/pyvenv.cfg", version_info: "3.12.0", interpreter: Runfiles { rpath: "rules_python++python+python_3_12_aarch64-apple-darwin/bin/python3", repo: "_main" }, user_site: false }
[aspect] "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/bin/python3"
[aspect] Attempting to execute: "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/external/rules_python++python+python_3_12_aarch64-apple-darwin/bin/python3" with argv[0] as "/private/var/tmp/_bazel_arrdem/6292cdb66616265d0039b3e456edaa70/execroot/_main/bazel-out/darwin_arm64-fastbuild/bin/app/app_bin.venv.runfiles/_main/app/.app_bin.venv/bin/python3" and args as ["-P", "-m", "app"]
hi from foo_lib
Hi from __main__
```

Author: arrdem

py/defs.bzl

@@ -93,6 +93,7 @@ def _py_binary_or_test(name, rule, srcs, main, data = [], deps = [], resolutions
 
     _py_venv_link(
         name = "{}.venv".format(name),
+        srcs = srcs,
         data = data,
         deps = deps,
         imports = kwargs.get("imports"),

py/tools/venv_shim/src/main.rs

@@ -12,16 +12,6 @@ use std::process::Command;
 const PYVENV: &str = "pyvenv.cfg";
 
 fn find_venv_root(exec_name: impl AsRef<Path>) -> miette::Result<(PathBuf, PathBuf)> {
-    if let Ok(it) = env::var("VIRTUAL_ENV") {
-        let root = PathBuf::from(it);
-        let cfg = root.join(PYVENV);
-        if cfg.exists() {
-            return Ok((root, cfg));
-        } else {
-            eprintln!("Warning: $VIRTUAL_ENV is set but seems to be invalid; ignoring")
-        }
-    }
-
     let exec_name = exec_name.as_ref().to_owned();
     if let Some(parent) = exec_name.parent().and_then(|it| it.parent()) {
         let cfg = parent.join(PYVENV);
@@ -216,13 +206,6 @@ fn main() -> miette::Result<()> {
         miette::bail!("could not discover an execution command-line");
     };
 
-    let (venv_root, venv_cfg) = find_venv_root(exec_name)?;
-
-    let venv_config = parse_venv_cfg(&venv_root, &venv_cfg)?;
-
-    // The logical path of the interpreter
-    let venv_interpreter = venv_root.join("bin/python3");
-
     // Alright this is a bit of a mess.
     //
     // There is a std::env::current_exe(), but it has platform dependent
@@ -305,7 +288,24 @@ fn main() -> miette::Result<()> {
 
     #[cfg(feature = "debug")]
     eprintln!("final  {:?}", executable);
-    let actual_interpreter = find_actual_interpreter(executable, &venv_config)?;
+
+    // Now that we've identified where the .runfiles venv really is, we want to
+    // use that as the basis for configuring our virtualenv and setting
+    // everything else up.
+    let (venv_root, venv_cfg) = find_venv_root(&executable)?;
+    #[cfg(feature = "debug")]
+    eprintln!("[aspect] venv root {:?} venv.cfg {:?}", venv_root, venv_cfg);
+
+    let venv_config = parse_venv_cfg(&venv_root, &venv_cfg)?;
+    #[cfg(feature = "debug")]
+    eprintln!("[aspect] {:?}", venv_config);
+
+    // The logical path of the interpreter
+    let venv_interpreter = venv_root.join("bin/python3");
+    #[cfg(feature = "debug")]
+    eprintln!("[aspect] {:?}", venv_interpreter);
+
+    let actual_interpreter = find_actual_interpreter(&executable, &venv_config)?;
 
     #[cfg(feature = "debug")]
     eprintln!(
@@ -347,6 +347,9 @@ fn main() -> miette::Result<()> {
     // Set the executable pointer for MacOS, but we do it consistently
     cmd.env("PYTHONEXECUTABLE", &venv_interpreter);
 
+    // Clobber VIRTUAL_ENV which may have been set by activate to an unresolved path
+    cmd.env("VIRTUAL_ENV", &venv_root);
+
     // Similar to `-s` but this avoids us having to muck with the argv in ways
     // that could be visible to the called program.
     if !venv_config.user_site {