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 T3874