origin_contributors: Use origin IDs instead of SWHIDs
Origin pseudo-SWHIDs are kind of useless on their own, as there is no public API to resolve them yet; so we might as well produce integers instead.
A future commit will add a mapping table to URLs, which are actually useful.
Part 1/2 to resolve swh-dataset#4729 (moved).
Migrated from D8970 (view on Phabricator)