pip - use project folder as installation source

Question

I would like to configure pip to use a custom search path to install packages from a local folder, which are not hosted on PyPI. The goal is to be able to run

$ pip install --user my_non_published_package

And have it install said package from /home/myuser/projects/my_non_published_package.

I know that I can explicitely install it using

$ pip install --user /home/myuser/projects/my_non_published_package

but this is not what I want. I want pip to transparently resolve potential depencies of other packages and install them from /home/myuser/projects/<package> iff they are not hosted on PyPI. So, if I have a project foo having a setup.py containing

from setuptools import setup

setup(
    ...
    install_requires=[
        bar,
        ...
    ],
    ...
)

And package bar is not on PyPI, I want pip to install it from /home/myuser/projects/bar automatically.

Update I found some information here, but the proposed solution does not work.

#! /usr/bin/env python3
"""Setup configuration."""

from pathlib import Path

from setuptools import setup


def local_pkg(name: str) -> str:
    """Returns a path to a local package."""
    return f'{name} @ file://{Path.cwd().parent / name}'

setup(
    name='openimmodb',
    use_scm_version={
        "local_scheme": "node-and-timestamp"
    },
    setup_requires=['setuptools_scm'],
    author='HOMEINFO - Digitale Informationssysteme GmbH',
    author_email='<info at homeinfo dot de>',
    maintainer='Richard Neumann',
    maintainer_email='<r dot neumann at homeinfo period de>',
    install_requires=[
        local_pkg('filedb'),
        local_pkg('mdb'),
        local_pkg('openimmo'),
        local_pkg('openimmolib'),
        'peewee',
        local_pkg('peeweeplus'),
        'pyxb',
        local_pkg('timelib'),
        local_pkg('xmldom')
    ],
    packages=[
        'openimmodb',
        'openimmodb.dom',
        'openimmodb.dom.ausstattung',
        'openimmodb.json',
        'openimmodb.json.ausstattung',
        'openimmodb.json.barrier_freeness',
        'openimmodb.json.flaechen',
        'openimmodb.json.immobilie',
        'openimmodb.json.preise',
        'openimmodb.mixins',
        'openimmodb.orm'
    ],
    entry_points={
        'console_scripts': [
            'oidbctl = openimmodb.oidbctl:main'
        ]
    },
    description='Relational OpenImmo database'
)

Results in:

...
Processing dependencies for openimmodb==0.1.dev987+g199a834.d20220110094301
Searching for xmldom@ file:///home/neumann/Projekte/xmldom
Reading https://pypi.org/simple/xmldom/
Couldn't find index page for 'xmldom' (maybe misspelled?)
Scanning index of all packages (this may take a while)
Reading https://pypi.org/simple/

Even though it is there:

 $ ls /home/neumann/Projekte/xmldom
LICENSE  Makefile  README.md  setup.py  venv  xmldom.egg-info  xmldom.py

Same with additional localhost:

Searching for xmldom@ file://localhost/home/neumann/Projekte/xmldom
Reading https://pypi.org/simple/xmldom/
Couldn't find index page for 'xmldom' (maybe misspelled?)
Scanning index of all packages (this may take a while)
Reading https://pypi.org/simple/

And with #egg=

Processing dependencies for openimmodb==0.1.dev987+g199a834.d20220110095236
Searching for xmldom@ file://localhost/home/neumann/Projekte/xmldom#egg=xmldom
Reading https://pypi.org/simple/xmldom/
Couldn't find index page for 'xmldom' (maybe misspelled?)
Scanning index of all packages (this may take a while)
Reading https://pypi.org/simple/

With the absolute path in install_requires I get:

$ python setup.py install
error in openimmodb setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers; Parse error at "'/home/ne'": Expected W:(abcd...)