Detect stubs from dependencies of targets

[chrisirhc]

This is a stab at #39 .

Logic mostly sourced from aspect-build/rules_py#235 .
This logic likely requires a recent version of mypy and has only been tested with the latest mypy version (1.10.0).

Main thing this introduces is to make imports of rules_python dependencies available to the mypy via PYTHONPATH. These are site-packages and are supported by mypy. mypy requires that these packages be installed, and not on MYPYPATH.

TODOs:

• Have not looked into how cache_map is used, I've commented it out, haven't investigated this. I could probably just follow the logic that's in Mypy aspect-build/rules_py#235
• Clean up the commits
• Address lint issues caught in CI
• Add a test for this behavior

[chrisirhc]

I believe this is an alternative implementation to #42

[chrisirhc]

I likely won't have bandwidth to take this to finish line.