Implement external repository mirroring using collections

From the milestone 2 requirements:

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.

This issue tracks the overall requirement, and will be broken down into several smaller issues.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information