[ty] don't allow first-party code to shadow stdlib types module

[carljm]

Summary

Fixes astral-sh/ty#741

The situation with shadowing types at runtime is a bit muddy. It's not a builtin module, and isn't explicitly non-shadowable. But in practice, it is typically imported very early in interpreter startup, and this means in most cases it practically can't be shadowed. Even in the scenarios where shadowing it is possible, this usually just causes interpreter startup to fail, due to trying to import things from it.

Since shadowing it doesn't really work at runtime, and also causes problems in type-checking (since we try to import things from it implicitly), the simplest approach is just to not support shadowing it.

Neither mypy nor pyright support shadowing it. Mypy has a nice diagnostic specifically informing you that you are trying to shadow it and that doesn't work; I added a TODO for us to consider adding this in future.

Test Plan

Added an mdtest which stack-overflowed before this PR.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

pytest-robotframework (https://github.com/detachhead/pytest-robotframework)
- TOTAL MEMORY USAGE: ~45MB
+ TOTAL MEMORY USAGE: ~49MB

mypy_primer (https://github.com/hauntsaninja/mypy_primer)
-     memo fields = ~41MB
+     memo fields = ~45MB

dulwich (https://github.com/dulwich/dulwich)
-     memo fields = ~129MB
+     memo fields = ~117MB

discord.py (https://github.com/Rapptz/discord.py)
- TOTAL MEMORY USAGE: ~251MB
+ TOTAL MEMORY USAGE: ~228MB

pydantic (https://github.com/pydantic/pydantic)
- TOTAL MEMORY USAGE: ~156MB
+ TOTAL MEMORY USAGE: ~142MB

isort (https://github.com/pycqa/isort)
- TOTAL MEMORY USAGE: ~80MB
+ TOTAL MEMORY USAGE: ~88MB

bandersnatch (https://github.com/pypa/bandersnatch)
-     memo fields = ~72MB
+     memo fields = ~66MB

psycopg (https://github.com/psycopg/psycopg)
- TOTAL MEMORY USAGE: ~228MB
+ TOTAL MEMORY USAGE: ~207MB

django-stubs (https://github.com/typeddjango/django-stubs)
- TOTAL MEMORY USAGE: ~189MB
+ TOTAL MEMORY USAGE: ~171MB

sphinx (https://github.com/sphinx-doc/sphinx)
- TOTAL MEMORY USAGE: ~304MB
+ TOTAL MEMORY USAGE: ~276MB

scipy (https://github.com/scipy/scipy)
- TOTAL MEMORY USAGE: ~1271MB
+ TOTAL MEMORY USAGE: ~1156MB

[sharkdp]

Thank you!

[AlexWaygood]

Vanquisher of stack overflows!