swh.indexer.storage: Refactor fossology license get
The content_fossology_license_get method in IndexerStorage now
returns a list of dictionaries whose keys are id and facts.
The facts key's value is a list of dictionaries. Each dictionary in
this list contains license information found for id by tool FOO.
Reviewed By: ardumont
Maniphest Tasks: T782
Differential Revision: https://forge.softwareheritage.org/D301