package.loader: Write to the ExtID storage

This allows future runs of a loader to know a package was already loaded, without querying each of the revisions individually and parsing their metadata.

Eventually, this will allow us to get rid of the 'metadata' column on the 'revision' table entirely.

Test Plan

They shall not pass, because they depend on swh-storage!662 (closed).

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Migrated from D5364 (view on Phabricator)