Draft: Sketch switching to new test/source targeting

These three patches are a sketch of how kpet-db could possibly look after we've redone our targeted source/test handling in kpet. Please see each commit and its message in turn, and leave feedback 😊 The names and mechanics are up to discussion, of course!

I'm not really sure this will work out exactly like this in the end, but it is something I'm aiming at for the start. Multiple places in kpet will need to be changed to support this, but this would allow us to fix #67 (closed), #62 (closed), kpet#47, and would probably help with kpet#48. This should also hopefully make the "targeting" behavior clearer and will get rid of nulls in patterns.