package/maven: Fix jar archive download after changes in lister
Maven lister has been updated to create one origin per package and group all its versions in it as releases.
Previously a maven origin URL corresponded to a jar archive URL while now it corresponds to the page where all versions of a package can be found.
So update loader implementation to reflect this change and fix jar archives download.
Also update tests after this change and refactor them a bit, notably by using fixtures instead of hardcoded tests data.
Related to swh-lister!420 (closed)
Related to T3874
Migrated from D7712 (view on Phabricator)