editable install breaks custom package_dir spec

I have a project with this directory structure:

- project_folder_name
  - lib
    - __init__.py
    - module.py
    - ...

So in my setup.py I followed the instructions in the docs and set the package_dir:

from setuptools import setup
setup(name='project-name',
      packages=['project'],
      package_dir={'project' : 'lib'},
)

If I install normally python setup.py install everything is fine and I can get the module like: import project. (Possibly another issue but pip install . seems to have issues though.)

However, in development mode python setup.py develop or pip install -e . I can't import via the package name. Strangely, I can import from the folder name import lib.

Just as a reference and sanity check I also tried other organizations that work i.e. "lib/project" with package_dir={'' : 'lib'}. Would like for this directory structure though if possible.

Reproduce the issue with the shown setup.py and the commands python setup.py develop and pip install -e ., then compare them with python setup.py install. Trace the editable-install handling for the custom package_dir mapping. Done means both editable-install paths allow import project rather than only import lib.