They should use ExtId instead.
(They already write the metadata that needs to be archived to `raw_extrinsic_metadata`)
* T3141 Make loaders pass ExtId objects between functions instead of "known_artifact"
* T3140 Make loaders use ExtId to find what arteficts are already loaded, in addition to revision.metadata
* T3139 Make loaders stop reading from the 'metadata' column of the 'revision' table
* T3138 Make loaders stop writing in the 'metadata' column of the 'revision' table