[ty] Check assignments to implicit global symbols are assignable to the types declared on types.ModuleType

[AlexWaygood]

Summary

Currently we'll happily let you do this without complaining about it:

__doc__ = 42

But this is unsound! All modules are instances of types.ModuleType, and typeshed states that all types.ModuleType instances have a __doc__ attribute of type str | None. Literal[42] is not assignable to str | None, so we should not permit this.

This PR fixes this soundness hole by falling back to declarations on types.ModuleType when checking assignments in the global scope. Cc. @tekknolagi (cf. #18071 (comment)).

I also fix an issue in this PR where we would incorrectly report a symbol as being unbound if it is declared as global in an inner scope and it is an implicit ModuleType global in the global scope, e.g.

def f():
    global __loader__
    print(__loader__)  # we previously emitted `[unresolved-reference]` here

Test Plan

mdtests added

[github-actions]

mypy_primer results

No ecosystem changes detected ✅

[carljm]

Looks good!

[AlexWaygood]

(Re-requesting review just for the changes pushed in e9b73f0)