[PEP 695] Fix nested generic classes

[JukkaL]

There was confusion about the fullnames of type variables in nested generic classes. A type variable could be defined internally as m.OuterClass.T, but it was sometimes accessed as m.T. The root cause was that the semantic analyzer didn't initialize the attribute that refers to the enclosing class consistently.

Fixes #17596. Fixes #17630.

[github-actions]

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

[svalentin]

lgtm! Just wondering if we need to worry about triple nested classes or some other odd cases.

[JukkaL]

Just wondering if we need to worry about triple nested classes or some other odd cases.

Multiply nested classes don't work with this PR, since lookup_fully_qualified_or_none doesn't support them. I'm leaving it out since they are probably quite rare, but I'll create a follow-up issue.