package.loader: Lookup packages from the ExtID storage

To check which packages are already downloaded.

For now, this lookup is done in addition to checking the artifacts from the last snapshot's revisions' metadata, because we did not start writing ExtIDs yet. But the ExtID lookup will eventually replace the artifact-based lookup.

This will finally allow us to drop the 'metadata' field of Revision objects.

Depends on !197 (closed)

Resolves #3140 (closed)

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Migrated from D5349 (view on Phabricator)