Fix crash in PEP 561 module resolution

[emmatyping]

If there was a py.typed file in a stub-only package, and it didn't
contain partial, there would be an unbound local error. To fix this,
we treat it as a normal stub-only package if the py.typed file doesn't
contain partial.

Fixes #5999

@wcooley this should fix the crash.

[gvanrossum]

Definitely need to cherry-pick this. "Be liberal in what you accept" etc. (https://en.wikipedia.org/wiki/Robustness_principle)

[gvanrossum]

Does it make sense to print a warning in the edge case? (Not sure if that would make things worse, since except if you're the package author, there's not much you can do about that warning.)

[emmatyping]

Yeah I thought about that, but if

[emmatyping]

Bah phones. The above should read:
"Yeah I thought about that but if we silently treat it as a normal package it will just not resolve the Python package, which we will report. Therefore the natural debugging check is to check if you included partial. Though maybe we should mention partial packages in the module not found message? "

[gvanrossum]

Oh, it would be easy to add that to https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports (where that message no redirects). So go ahead and merge now!