Allow using modules as subtypes of protocols

[ilevkivskyi]

Fixes #5018
Fixes #5439
Fixes #10850

The implementation is simple but not the most beautiful one. I simply add a new slot to the Instance class that represents content of the module. This new attribute is short lived (it is not serialized, and not even stored on variables etc., because we erase it in copy_modified()). We don't need to store it, because all the information we need is already available in MypyFile node. We just need the new attribute to communicate between the checker and subtypes.py.

Other possible alternatives like introducing new dedicated ModuleType, or passing the symbol tables to subtypes.py both look way to complicated. Another argument in favor of this new slot is it could be useful for other things, like hasattr() support and ad hoc callable attributes (btw I am already working on the former).

Note there is one important limitation: since we don't store the module information, we can't support module objects stored in nested positions, like self.mods = (foo, bar) and then accepts_protocol(self.mods[0]). We only support variables (name expressions) and direct instance, class, or module attributes (see tests). I think this will cover 99% of possible use-cases.

[ilevkivskyi]

Hm, those few errors are clearly wrong, and I know where they are coming from. Will try to fix now.

[ilevkivskyi]

Yes, I think both should be errors. I will add tests later today (and update implementation if needed).

[ilevkivskyi]

@sobolevn OK, I added test and fixed code. Btw I discovered that we didn't do the final check for normal instances either, so I fixed that as well (I actually even found an issue about this on the tracker).

[sobolevn]

🎉

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[gvanrossum]

You rock! Also thank you to the reviewers.

[ilevkivskyi]

Got some free time on my paternity leave :-)

[gvanrossum]

Got some free time on my paternity leave :-)

Oh congratulations!

[ilevkivskyi]

Thank you!