cran.loader: Align cran loader with other package loaders
Prior to this commit, the origin url was targeting solely one versioned artifact. This adaptation allows to have a canonical url as origin to which we associate a list of artifacts to ingest (symmetrically as what is done for other package loaders).
Related to swh-lister!133 (closed)
Test Plan
tox
Migrated from D2532 (view on Phabricator)