autodoc: Allow TypeVars to be reimported from other modules

[coretl]

Purpose

To allow autodoc to read the doc comments of TypeVars re-imported from other modules. This allows TypeVars from private modules to be documented there and re-exported as part of the public interface.

References

Fixes #13276

[coretl]

I thought I had fixed it but apparently not. This PR solves half the problem, getting the doc comments from the private module to the public module. Unfortunately all uses of the TypeVar result in a signature that references its original private module, which Sphinx can not resolve. I'm not sure how to solve this, ideally when automodule finds a TypeVar it should tell Sphinx that when it sees a reference to it's private __module__ it should resolve the public location that it published.

I will leave this as draft in case someone else has any ideas, and have put a workaround in the original issue.