This issue is actually about the fact that 1.3.0 treats the initial argument specially - one bad effect is that custom collectors like the codecheckers collector and the internal module collector may not both be collected if the file is directly specified.
refactor all collection related logic
- drop all pickling support (for now)
- perform collection completely ahead of test running (no iterativity)
- introduce new collection related hooks
- shift all keyword-selection code to pytest_keyword plugin
- simplify session object
- besides: fix issue88