ModuleNotFoundError with setup.py using a compiled pyc module

I can normally import a compiled .pyc module as well as .py, but when trying to package a simple project with setup.py, I'm getting the ModuleNotFoundError exception for the compiled .pyc module. Because this is only happening when using setup.py, otherwise is working fine, I don't know if there's something I should had to setup.py to make this work.

The project structure is currently something like this:

proj
├── FAILING.pyc
├── __init__.py
├── aux/
│   ├── __init__.py
│   └── aux.c
└── main.py

and the setup.py:

from setuptools import setup, Extension, find_packages

DISTNAME = 'proj'

INSTALL_REQUIRES = [
        'cython>=0.29.13',
        'numpy>=1.16.4'
]
PYTHON_REQUIRES = '>=3.6'

ENTRY_POINTS = {
    'console_scripts': ['proj = proj.main:main']
}

def setup_extensions(metadata):
    ext_modules = [Extension('proj.aux.aux', sources=['proj/aux/aux.c'])]
    metadata['ext_modules'] = ext_modules

def setup_package():
    metadata = dict(
        name=DISTNAME,
        version='0.1',
        package_dir={'': '.'},
        packages=find_packages(),
        entry_points=ENTRY_POINTS,
        python_requires=PYTHON_REQUIRES,
        install_requires=INSTALL_REQUIRES,
        zip_safe=False,
    )
    setup_extensions(metadata)
    setup(**metadata)


if __name__ == '__main__':
    setup_package()

The main.py:

#!/usr/bin/env python3

import proj.aux.aux as aux
import proj.FAILING

def main():
    print('Hello World')

If I just try to import FAILING.pyc on the repl everything works as expected:

>>> import FAILING
>>>

But if I first run python3 setup.py intall and then call proj I'm getting the following error:

$ proj
Traceback (most recent call last):
  File "/path/to/bin/proj", line 11, in <module>
    load_entry_point('proj==0.1', 'console_scripts', 'proj')()
  File "/path/to/lib/python3.8/site-packages/pkg_resources/__init__.py", line 489, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/path/to/lib/python3.8/site-packages/pkg_resources/__init__.py", line 2852, in load_entry_point
    return ep.load()
  File "/path/to/lib/python3.8/site-packages/pkg_resources/__init__.py", line 2443, in load
    return self.resolve()
  File "/path/to/lib/python3.8/site-packages/pkg_resources/__init__.py", line 2449, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/path/to/lib/python3.8/site-packages/proj-0.1-py3.8-macosx-10.14-x86_64.egg/proj/main.py", line 4, in <module>
    import proj.FAILING
ModuleNotFoundError: No module named 'proj.FAILING'

I'm also running this inside a virtualenv environment, although I'm guessing this is not related to the error.

What am I doing wrong, or what would I need to change to make this work?

from ModuleNotFoundError with setup.py using a compiled pyc module