splt ddbc into sub packages

[SingingBush]

as per https://github.com/buggins/ddbc/pull/92#issuecomment-770203976

I think it's worth having the project split into sub packages, so there's something like:

ddbc:core ddbc:drivers ddbc:pods (or perhaps ddbc:uda?)

initially just so that the pod feature isn't bundled in by default but as the code grows this will help to keep it from being too tightly coupled.