Design collections

We need a design for collections. This needs to support the following activities from M2:

• MUST: Implement a way to mirror an external repository in debusine, building on a new concept of “collection”
  • Come up with a sane and generic database models to represent a repository
    • Possibly a hashed “collection” of artifacts with keys like “debian:source-package:coreutils_1.2.3-1” or “debian:binary-packages:coreutils_1.2.3-1_amd64” and values pointing to Artifact identifiers.
    • Check how aptly does it
  • Implement a FileStore that makes the content of an external repository available within debusine
  • Mirror operation might be a workflow step.
• MUST: Have a mechanism that triggers tasks to update a “collection” so it is kept in sync with another collection (add missing keys, drop outdated keys)
  • That mechanism might be a workflow step.
  • Design of atomic updates ? Naming of collections ?

I've made a start on this in !427 (merged), but I'm also belatedly filing this issue so that we can add it to the M2 milestone.