Load plugins from paths in 'pythonpath' option

[millerdev]

Real fix for #11118

[millerdev]

Can't figure out how to resolve the codecov failure, which seems to be illegitimate anyway since it removed the incorrect "not covered by tests" annotations. Otherwise, this is ready for review.

[bluetech]

@millerdev Since the issue is closed, could you explain your use case for this change? I'm asking because we generally prefer to put functionality in plugins, so we'd like to understand the reasoning before moving to the core.

[millerdev]

@bluetech I am trying to load a plugin that implements early hooks like pytest_load_initial_conftests from within the package under test. I could find no other way to do it without this change. My requirements are:

• tests must be run against packages in the CWD (pythonpath contains . to enable this)
• tests must be run with the pytest command (not python -h pytest).
• a custom pytest plugin required to run the tests resides within one of the packages under test
• the packages under test are not pip install-able

The python_path plugin where this functionality was implemented was using the earliest available pytest hook (pytest_load_initial_conftests) to update sys.path with values from the pythonpath option. Unfortunately that hook is not early enough in the plugin lifecycle to allow plugins to be loaded from pythonpath paths.