swh.indexer.storage: Refactor fossology license get

Summary:
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.

Related T782

Reviewers: ardumont

Reviewed By: ardumont

Subscribers: ardumont

Maniphest Tasks: T782

Differential Revision: https://forge.softwareheritage.org/D301