Reorganize the code

- move everything (swh)archive related in a archive/ submodule
- move everything provenance storage related in a storage/ submodule
  (which remains a not ideal name, may be confusing with the general
  'storage == swh-storage' acceptance in swh)
- rename rabbitmq's backend from api/ to storage/rabbitmq
- spit interface.py in 3 parts (one for each part, ProvenanceInterface,
  ProvenanceStorageInterface and ArchiveInterface).