Why does pytest not discover my fixtures? - import

Question

I'm having trouble with pytest not discovering fixtures. The situation is a bit messy, but I'll do my best to explain. I expect this question to be incomplete initially, so I'm hoping that we can work through it together.
I have a couple of git repos. In each of them, I use nox to (among other things) set up virtual environments and run tests with pytest.
Previously, I had logic in repo A that was unit tested there. This logic was then imported in repo B and used to define fixtures there. Those fixtures were then used in tests in repo B and elsewhere.
Now, I want to bundle the fixtures and the logic in repo A, test the fixtures there, and expose it as plugin for use in repo B.
The trouble I'm having is that the tests that I've moved from repo B to repo A cannot discover the fixtures.
What is the right way to do this?
Below is an attempt to show only the relevant parts of my current situation. Please ask if something is not clear.
repo A
-setup.py
-/repo_a
--__init__.py
--/pytest
---__init__.py
---config.py
---fixture.py
-/tests
--__init__.py
--conftest.py
--test_fixture.py
repo_a/pytest/fixture.py
import pytest
from repo_a.foo import bar
#pytest.fixture
def fixture_a():
bar.pass()
repo_a/pytest/config.py
pytest_plugins = ["repo_a.pytest.fixture.py"]
tests/confest.py
pytest_plugins = ["repo_a.pytest.config"]
tests/test_fixture.py
def test_fixture_a(fixture_a):
fixture_a.pass()
When running pytest tests/test_fixture.py this fails in the setup phase, with
ImportError: Error importing plugin "repo_a.pytest.config": No module named 'repo_a.pytest'
I would have expected this if there was no __init__.py in the pytest folder, but there is.