Consider merging core repositories and packages into one Python package

Currently we have multiple repositories we are having separate:

• metadata
• primitive-interfaces
• d3m
• tests-data

This is mostly historic separation also to help with coordination of different working groups. But it is introducing some issues: one has to be precise to have correct combination of versions installed for them to work together, there are some cyclic dependencies between them, and it is also a bit confusing for development where you need to do a change, or make an issue, or make a merge request. Especially if you are not familiar with all codebase.

So we could merge all those repositories sand packages into on package, maybe d3m. This would then also mean that interfaces are merged with standard code implementations, but it seems that they are already strict dependencies here so maybe this is not a problem.

This would be a breaking change requiring others to adapt their code.