Track classes with explicit __module__ attributes

[tungol]

Having access to this information can be used for more accurate diagnostics and tests

At runtime, classes which declare their __module__ attribute explicitly have that reflected in
their repr. This MR stores this information on TypeInfo nodes as the module_override attribute.
The original motivation for that is this typeshed discussion python/typeshed#11141

This is a relatively minimal change to enable the desired behavior. In trying to understand how this part of mypy works, I considered whether it was possible to store the module name and qualname separately and only generate the fullname on demand, like runtime does. I quickly discovered how much state is indexed by the fullname, so that seems like it'd be a massive project.

I was originally looking at modules which set their __name__ attribute explicitly. That's similarly easy to capture, but the number of places that would need to propagate to was a bit intimidating.

Strictly speaking, we don't need the qualname property, but it seemed reasonable to make that available since I had to generate it anyway. Changing TypeInfo.dump() to use the new realname property was done solely for the sake of being able to write a test.

I don't love realname as the name for this property, but I couldn't think of anything better. Real in the sense that that's what runtime would say it's name is.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅