py: resolve module paths without executing modules

[FFY00]

Fixes #57

[virtuald]

This seems like a better way to solve the problem, but I don't have time to run it locally. Since you actually wrote tests, I suspect it works. :)

[virtuald]

Hm, this is an interesting approach. I can't think of a good reason why not to do it, but it smells really fishy. I presume that this is to ensure that any side effects of loaders aren't kept around either, but... it feels mildly problematic and racy, but maybe that's fine.

[FFY00]

Yeah, we need it because we don't want to leave an unexecuted module in sys.modules. Initially I wanted to refrain from writting anything to sys.modules at all, but unfortunately NamespacePath looks for the parent in sys.modules when we access __path__.

https://github.com/python/cpython/blob/1b39b502d33c68f52fd775c4e6c2174baddd40bd/Lib/importlib/_bootstrap_external.py#L1112

That said, I remembered about https://docs.python.org/3/library/importlib.html#importlib.util.LazyLoader, which instead of executing the module in exec_module, it changes the module class into a custom type that postpones execution until an attribute is accessed. Unfortunately, it still doesn't let us access the types.ModuleType attributes without executing, but this can be overcome with a custom implementation.