Add component called lib (?) that differentiates from lower level, flexible core

[lwjohnst86]

As I have been writing and thinking about the data resource side of things, plus with some comments @martonvago made about knowing when to assume certain path locations or not, some things felt a bit "ragged"/"rough". I thought, taking inspiration for how the CLI might behave, maybe it would be better to have a very low level, very flexible, less assumption-based (not dependent on specific path locations at all) core and a higher level, more opinionated and assumption based lib for working with data packages and our data organization in Python (not as the CLI).

I'll write a draft in the design section to make these thoughts clearer.