`packages_distributions` relies on setuptools implementation detail

[konstin]

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