python_toolchain: set and use preprocessor flags for extensions

[zsol]

This PR:

• adds a new extension_preprocessor_flags field to PythonToolchainInfo
• starts using it from the cxx_python_extension rule
• wires the new field up from remote_python_toolchain
• moves the python_cextension targets from under with_prelude/cpp to with_prelude/python
• enables them so CI starts covering these
• removes the main() function from the extension cpp file

At the end of this all, remote_python_toolchain (and so the demo toolchains) is now capable of building Python C extensions without having Python installed on the host at all.

Test plan:

buck2\examples\with_prelude  "python_toolchain: set a…" [8 +22-58]
❯ ..\..\target\release\buck2 run python/use_cextension:cext
Build ID: fe1005ad-8885-47c3-9d70-830a65addf98
Network: Up: 0B  Down: 0B
Command: run.
Time elapsed: 0.0s
BUILD SUCCEEDED - starting your binary
hello from cpython extension from cpp toolchain

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[zsol]

@itamaro wdyt?

[itamaro]

lgtm!

one naming nit -- I guess extension_preprocessor_flags is technically accurate, but perhaps it would be clearer to users to call this extension_compiler_flags? or even cxx_extension_compiler_flags, since this wouldn't make sense for, e.g., a rust python extension?