Finalize nixguix loader implementation

• !71 (closed): Support files that are not archive but plain text (stand-by)
• Use all mirrors instead of just the first one.
• Implement a mechanism to subscribe to a list of sources.json.
• Support others types of source (git for instance)...

Related to T1352 Related to #1991 (closed)

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Migrated from T2879 (view on Phabricator)