feature: Warning/hint for objects coming from siblings/parents/external exposed in __all__

[pawamoy]

Is your feature request related to a problem? Please describe.

I noticed in my own code that I exposed an object coming from a sibling module in __all__.
I consider this bad practice, and would like a way to warn me if something like this is detected by Griffe.

Describe the solution you'd like

When visiting __all__ assignments or when expanding exports, check that exposed objects were either declared in this module, or in a submodule.

Describe alternatives you've considered

Additional context