maven: Consistently read lister input to ingest a mvn origin
Prior to this commit, the test data were in the wrong format. They were passing along an url within the artifact parameter which is not provided by the lister.
This fixes the inconsistency.
Bonus point, by adapting the loader this way, there is no need to tamper with the listed_origins data table (scheduler).
Related to T3874
Test Plan
tox
Migrated from D7573 (view on Phabricator)