Declaring marks used by the defining package and other packages

[jwodder]

We have a package (call it cheese) that contains (in addition to the primary code) various test-helper functions & utility decorators that, among other things, apply custom markers to decorated tests. Various other packages that depend on cheese make use of these helpers in their own tests, but because the markers are (currently) only defined in the tox.ini file in cheese's repository, warnings about unknown markers are emitted when testing these other packages. The best way to solve this appears to be to declare the markers in a pytest_configure hook, but that leads to the following questions:

1. Is the only way to make the pytest_configure hook take effect for other packages to turn cheese into a pytest plugin?

2. Where should we put the pytest_configure hook so that it takes effect for cheese's own tests? Note that cheese's conftest.py file contains some autouse fixtures which we do not want used by other packages, so I expect that putting the hook in that file and pointing the pytest plugin to it would cause problems.

[yarikoptic]

ping @pytest-dev team on this question