When installing a package using --editable, pip creates a .egg-link file in your site-packages dir that points to the .egg-info metadata that by default exists along side the source that it was installed from. This worked just fine with the older pkg_resources package. However, the newer importlib.metadata does not seem to be aware of .egg-link files at all. The only solution is to include the source dir that contains the .egg-info metadata into your PYTHONPATH, which sort of defeats the purpose of having the .egg-link pointer in the first place.

I should also add that the easy-install.pth file, which similarly contains a link to the source dir containing the .egg-info metadata, is also not processed to locate the necessary metadata.

It would make sense if importlib only supported official specs.
For editable/develop installs, it is https://www.python.org/dev/peps/pep-0660/ which does not mention egg (not egg-link nor egg-info, long since replaced by dist-info).

Are you using latest pip, and pyproject.toml?  I would expect pip to follow the PEP too.

yes, the latest version of pip creates an .egg-info metadata dir at the level of the package src dir and .egg-link and easy-install.pth stubs in the site-packages dir (the contents of which are paths to the package src dir). The intent is that these links would be followed to find the necessary metadata for the package. However, importlib.metadata does not follow either one of these links. These links are what makes editable installs possible, so I'm not sure how they can be ignored all together, unless there's some different facility that provides this functionality.

https://github.com/python/importlib_metadata/issues/364