fix: __all__ references are not ignored

[vantaboard]

Summary

Creating a RemoveUnusedImports transformer according to the docs does not properly handle the __all__ variable.

Test Plan

I added a test to libcst/metadata/tests/test_scope_provider.py that ensures the imports have references that target the strings in __all__.

[zsol]

I wonder what happens for this snippet:

import a, b
__all__: "list[str]" = ["a", "b"]

[zsol]

I would add an entry into self.__deferred_accesses instead, to make sure this Access gets the same treatment as any other.

[zsol]

Gotta fix the type errors (looks like Name.__init__ needs updating too) and this is good to go!