Implement a Guix/Nix loader

The objective of this task is to create a loader for Nix/Guix distributions. These distributions expose a sources.json containing all sources used at a specific moment.

This work could be split into the following diffs:

• !352 (closed): Create a loader able to load archives
• !353 (closed): Add a pointer in the snapshot to the commit used to generate the sources.json file
• #2411 (closed): Deploy current nixguix loader version (fix issues along the way)

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Migrated from T1991 (view on Phabricator)