Implement support for non-core.py Python rulebases

[SwooshyCueb]

We should implement support for non-core.py rulebases in PREP. There are several TODOs in the code for this, but I don't see an open issue.

[trel]

core.py has proven enough of a jumping off point...

and with only one way to do things... it's fewer configuration options to hold poorly.

so yes, if nothing else, we can remove the TODOs.

[trel]

Ah, if there are multiple instances... each instance could load a separate .py module.

But I think we should limit the config to only allow one file per instance... and still core.py if empty/undefined so it remains backwards compatible.