Improve API selector / lookup organization

After the migration to the V1 version of the API we have notions of a selector (the set of parameters required to lookup a resource) and a lookup (the mechanism by which the resource is resolved). An emergent notion of this design is that selectors themselves are only intended to be used to resolve lookups inside http endpoint handlers and that the handlers are the only thing that should be passed to the app layer. This notion is muddied a bit by the fact the functions that actually perform lookups themselves live in the app layer. We should do some cleanup to make this distinction clearer.

This mainly entails moving lookup functions outside of the app layer and into their own module.