cpan: Align loader implementation with latest lister improvements
Artifacts info for a package are now provided as loader arguments so no need to query metacpan Web API anymore.
Also remove package description in each release message as it does not describe a release but rather the package.
Related to swh-lister#2833
Depends on swh-lister!321 (closed) Depends on !331 (closed)
Migrated from D8616 (view on Phabricator)