support the loading of metadata-only deposits in the metadata storage

The metadata deposit is different from the content metadata in its loading process, since the metadata is kept in the metadata storage.

There is no loading process to the storage and there aren't any artifacts created in the graph.

We discussed the subject of creating an origin, for example a HAL url for the deposited metadata and the difficulty to search with the archives-ouvertes.fr domain. But we have decided that there is no need for an origin.

For the corner case scenario where a user creates a v2 for an existing v1 deposit: there is still a discrepancy with the development history construction.

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Migrated from T2540 (view on Phabricator)