cran lister: Align lister to output list of tarballs per origin

The lister currently list one (versioned) tarball at a time (filtered on some fields) It must be possible to aggregate and group those tarballs (output by the R script) by project/origin.

That way, we could share some common behavior between the gnu/cran loaders (1 origin with list of tarballs). As hinted at in swh/devel/swh-loader-core!369 (closed).

The output of the lister should also become "oneshot" scheduler tasks.

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Migrated from T2029 (view on Phabricator)