warning "Module x was never imported" when running with xdist (reopen)

[cjolowicz]

This may be the same issue as #212. If you prefer to reopen that issue, I can repost there.

When using pytest-cov with pytest-xdist, Coverage.py displays the warnings module-not-imported and no-data-collected, even though coverage does get collected and reported:

$ pytest --cov=foobar -n auto

=== test session starts ===
platform darwin -- Python 3.8.1, pytest-5.3.2, py-1.8.1, pluggy-0.13.1
rootdir: /private/tmp/foobar
plugins: xdist-1.31.0, forked-1.1.3, cov-2.8.1
gw0 [1] / gw1 [1] / gw2 [1] / gw3 [1]
.   [100%]
Coverage.py warning: Module foobar was never imported. (module-not-imported)
Coverage.py warning: No data was collected. (no-data-collected)


--- coverage: platform darwin, python 3.8.1-final-0 ---
Name                                                   Stmts   Miss  Cover
---
.venv/lib/python3.8/site-packages/foobar/__init__.py       1      0   100%

=== 1 passed in 0.98s ===

Repro: https://github.com/cjolowicz/pytest-cov-issue-378

git clone https://github.com/cjolowicz/pytest-cov-issue-378.git
cd pytest-cov-issue-378/
pyenv local 3.8.1
python -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt
pip install .
pytest --cov=foobar -n auto

[cjolowicz]

For the record, configuring Coverage.py does not seem to help:

# .coveragerc
[paths]
source =
    src
    */site-packages

[run]
branch = true
source = foobar
concurrency = multiprocessing
parallel = true