Use directory metadata in origin search

Metadata from deposits is archived as REMD objects targeting a directory, with an origin as context.

We should use that origin when indexing, so swh-search can do something with it.

Ideally, swh-search should be directory-aware and swh-graph should provide reverse-traversal back to the origin, but we are still a long way from this. This workaround should be pretty easy to implement in the meantime.

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Migrated from T4694 (view on Phabricator)