You can achieve what you want without pytest-cov.

❯ coverage run --source=<package> --module pytest --verbose <test-files-dirs> && coverage report --show-missing

OR SHORTER

❯ coverage run --source=<package> -m pytest -v <test-files-dirs> && coverage report -m

Example: (for your directory structure)

❯ coverage run --source=plugin_module -m pytest -v tests && coverage report -m

======================= test session starts ========================
platform darwin -- Python 3.9.4, pytest-6.2.4, py-1.10.0, pluggy-0.13.1 -- /Users/johndoe/.local/share/virtualenvs/plugin_module--WYTJL20/bin/python
cachedir: .pytest_cache
rootdir: /Users/johndoe/projects/plugin_module, configfile: pytest.ini
collected 1 items

tests/test_my_plugin.py::test_my_plugin PASSED               [100%]

======================== 1 passed in 0.04s =========================
Name                            Stmts   Miss  Cover   Missing
-------------------------------------------------------------
plugin_module/supporting_module.py  4      0   100%
plugin_module/plugin.py             6      0   100%
-------------------------------------------------------------
TOTAL                              21      0   100%

For an even nicer output, you can use:

❯ coverage html && open htmlcov/index.html

Documentation

❯ coverage -h
❯ pytest -h

coverage

run -- Run a Python program and measure code execution.

-m, --module --- Show line numbers of statements in each module that weren't executed.

--source=SRC1,SRC2, --- A list of packages or directories of code to be measured.

report -- Report coverage stats on modules.

-m, --show-missing --- Show line numbers of statements in each module that weren't executed.

html -- Create an HTML report.

pytest

-v, --verbose -- increase verbosity.

if you do not specify --cov=/path/to/code then it will not generate the html at all.

$ py.test --cov-report html test_smoke.py
== test session starts == 
platform linux2 -- Python 2.7.12, pytest-3.4.0, py-1.5.2, pluggy-0.6.0 rootdir: /home/someuser/somedir, inifile: plugins: xdist-1.22.0, forked-0.2, cov-2.5.1 collected 3 items                                                                 


test_smoke.py ...                                             [100%]

== 3 passed in 0.67 seconds ==

We can see that there is no message that output was created... However if we specify --cov=...

$ py.test --cov-report html test_smoke.py --cov=/path/to/code
== test session starts ==
platform linux2 -- Python 2.7.12, pytest-3.4.0, py-1.5.2, pluggy-0.6.0
rootdir: /home/someuser/somedir, inifile:
plugins: xdist-1.22.0, forked-0.2, cov-2.5.1
collected 3 items                                                                                                                                                                                                                                                         

test_smoke.py ...                                            [100%] 

---------- coverage: platform linux2, python 2.7.12-final-0 ----------
Coverage HTML written to dir htmlcov

We now see that there are no stats for tests that passed, instead we see that coverage was written to HTML and sent to the default directory: ./htmlcov

NOTE: if you want a different directory, then affix :/path/to/directory to the output style html -> py.test --cov-report html:/path/to/htmldir test_smoke.py --cov=/path/to/code

If you see a plain html file, this is an indication that your problem is the --cov=/path/to/my/pkg perhaps... are you sure that the code you are testing lives here?