Support PEP 582: __pypackages__

[brettcannon]

https://peps.python.org/pep-0582/

[brettcannon]

I think #168 has enough info to cover what the PEP requires to calculate the path. Right now the PEP says to use sysconfig._INSTALL_SCHEME['posix_prefix'] which defaults to '{platbase}/{platlibdir}/python{py_version_short}/site-packages for CPython and '{platbase}/lib/{implementation_lower}{py_version_short}/site-packages' for PyPy. You get {py_version_short} from sys.version_info and {implementation_lower} from sys.implementation.name, both of which are covered in the locator proposal.

I also think PYTHONPATH could be used to inject __pypackages__ since it always comes after the current directory but before anything that's put on to sys.path by default.

[brettcannon]

And a shortcut, if the key is made to be the strictest path instead of the most broad as it is right now, is to compare to the file name to the directories under __pypackages__/lib.

[brettcannon]

The SC rejected PEP 582, so I won't be implementing support.