Fixed a frustrating bug that afflicted some users with the only error being assert mod not in mods. The issue was caused by the fact that str(Path(mod)) and mod.__file__ don't necessarily produce the same string, and was being erroneously used interchangably in some places in the code.
This fix also broke the internal API of PytestPluginManager.consider_conftest by introducing a new parameter -- we mention this in case it is being used by external code, even if marked as private.