embrace repository snapshot object in the data model (meta task)

(Multiple) origins might expose the very same state of a repository over time. To both avoid bloat and be more coherent with the "big Merkle DAG" approach of the data model, we want to introduce a new kind of node in our DAGs that capture the current state of a repository and associate to it a content-adressable identifier.

These new Merkle DAG nodes will be called repository snapshot objects. A more formal description of repository snapshots is available in repository-snapshot-object.md and on the wiki.

This meta task tracks the various changes needed throughout Software Heritage to fully embrace repository snapshot objects.

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Migrated from T565 (view on Phabricator)