chore(stubgen): __all__ correctness

[Rayahhhmed]

Context

When a Python module defines __all__, it explicitly declares its public API. Before this change, pyrefly stubgen ignored __all__ entirely and relied solely on naming conventions (skip _ prefixed names which is correct).

However, this meant that internal helpers not in __all__ leaked into the generated stub, and private names explicitly listed in __all__ could be incorrectly filtered out.

For example, given:

__all__ = ["public_func", "PublicClass"]
def public_func(x: int) -> str: ...
def unlisted_func() -> bool: ...  # internal, not in __all__
def _explicitly_exported() -> None: ...  # private by convention but in __all__

Before: unlisted_func appeared in the stub (wrong), _explicitly_exported was excluded (wrong).

After: Only public_func and PublicClass appear and this is exactly what __all__ specifies.

Approach

• Import statements are always preserved regardless of __all__, since they may be needed for type resolution in the stub.
• Type aliases (TypeVar calls, old-style aliases, PEP 695 type statements) are now also filtered through should_include_name, which they previously bypassed.
• Threads the result through ExtractionContext into should_include_name(), which short-circuits at module level: if __all__ is present, only listed names pass. Inside classes, __all__ is irrelevant and the existing convention logic applies unchanged.
• Falls back gracefully to convention-based filtering when __all__ is absent, unresolvable, or contains cross-module re-exports.

Test plan

• New snapshot test: dunder_all
• All 11 existing stubgen snapshot tests pass and no regressions for modules without
  __all__
• python3 test.py --no-test --no-conformance passes

[meta-codesync]

@yangdanny97 has imported this pull request. If you are a Meta employee, you can view this in D96861208.

[yangdanny97]

@migeed-z why is mypy primer showing a huge delta here? this PR should be a no-op

[migeed-z]

@migeed-z why is mypy primer showing a huge delta here? this PR should be a no-op

The fix landed. It's looking normal again on new PRs.

[Rayahhhmed]

Rebased on main again! Thanks @migeed-z :)