`packages_distributions` relies on setuptools implementation detail

This is a follow-up to https://github.com/python/importlib_metadata/issues/402.

packages_distributions reads top_level.txt to determine the names. This file isn't part of any PEP and to my knowledge only implemented by setuptools, as a historical artifact from eggs. This leads users to believe that build backends not supporting this function have a bug (https://github.com/astral-sh/uv/issues/16423).

Would it be possible to update the documentation to accurately reflect that this is not a problem with some PEP 660 build backends not supplying top-level names, but that this function is generally not expected to work with editable installs at all, with the historical exception of setuptools?

(This may change with PEP 794)

https://github.com/python/importlib_metadata/blob/95ecf2aa573a68d4624506f801ca70992a337f41/importlib_metadata/__init__.py#L1133-L1134

Inizia leggendo la documentazione di packages_distributions e la sezione collegata di importlib_metadata/init.py. Aggiorna la documentazione per spiegare che in genere non ci si aspetta che le installazioni modificabili forniscano nomi di primo livello, con setuptools come eccezione storica, indicando al contempo il possibile impatto futuro di PEP 794.