Make 'type' an attribute of origin visits, not origins

So origin == url, and different loaders can visit that same origin.

The plan:

• Add 'type' to origin visits swh/devel/swh-storage!211 (closed)
• Make all clients use origin visit types instead of origin types
  • loaders
  • listers
  • swh-web
  • others
• Drop 'type' from origin

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Migrated from T1891 (view on Phabricator)