This is a fairly common packaging mistake that it's better that we solve in a hook than in one-by-one in overrides.
Normal files in site-packages isn't always a mistake, so we can't clean them up wholesale, but we can clean up some common mistakes.
We should never have to use `format = "setuptools"` since pip can
_also_ handle setuptools packages.
This uses a custom pipBuildHook that removes pyproject.toml in case
there is also a setup.py in the same sources.