Add support for more forms of computed `__all__`

[AustinScola]

Feature Add typing support for the following forms of computing __all__:

__all__ += ['a', b']
__all__ += submodule.__all__
__all__.extend(submodule.__all__)

Pitch Combining __all__ from multiple submodules is a common and useful pattern.

Related: https://github.com/python/mypy/issues/12582

[rayzchen]

What is the progress of this?

[sshishov]

When we can expect it to be fixed? Is someone working on it? The initial setup is working, but only for 1 layer! If you add more layers, then it is breaking.

[warsaw]

I'll also point out that @public is another common-ish way of computing __all__ and it also causes a raft of bogus attr-defined warnings. E.g.

from public import public

@public
def a_public_method():
    pass

@public
class APublicClass:
    pass

from foo.bar import PublicFoo, PublicBaz

# Often seen in a re-exporting __init__.py
public(
    PublicFoo=PublicFoo,
    PublicBar=PublicBar,
)

All of these cases are common ways to declare publicly exported names for __all__ and which are not supported by mypy.