Fix crash on invalid recursive variadic alias

[ilevkivskyi]

Fixes #21125

This one was tricky. This is because the above issue actually exposed two different crash scenarios:

• A crash on invalid constructs like *tuple[Ts] (must be *tuple[*Ts]).
• An infinite recursion when trying to detect pathological and divergent aliases.

And while working on this I discovered two more cases:

• A crash where an invalid type (like a union) appears in unpack in a recursive alias definition.
• A crash on non-normalizeable recursive tuple.

I fix the first by tightening logic in typeanal.py w.r.t. where exactly a TypeVarTuple is allowed. I fix the second and third by avoiding get_proper_type() calls in expand_type() for recursive tuples. The fourth is the most problematic, and is kind of a fundamental thing. This PR only avoids an immediate crash for such aliases. We will still need to update various call sites where we special-case tuples to expect non-normal ones.

Couple more related things:

• I fix couple issues with is_recursive cache invalidation.
• I added a fast path to detect_diverging_alias() to avoid creating sets unless really needed.

[github-actions]

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