Migrate revision metadata to extid in the storage

We want to make loaders read from the ExtID storage instead of 'revision.metadata'. We could do this without migrating the data, but it means loaders will be slow for a while, as they would lose the memory of what packages they already loaded.

This means we should read all the existing revision metadata, and generate ExtIDs before getting rid of the 'revision.metadata' column.

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Migrated from T3143 (view on Phabricator)