Implementation of a lister started at D2025.

First runs triggered on staging, errors will be reported in sentry [1]

[1] https://sentry.softwareheritage.org/organizations/swh/issues/?environment=staging&project=9&query=is%3Aunresolved&statsPeriod=1h

For information, this has been running on staging daily for a while now without much issues. The sources.json are nix specifics for now.

@lewo decreased the number of potential errors [1] in the current loader by
removing potential issues in the sources.json [2]

Latest run on staging:

May 17 10:55:39 worker0 python3[1066]: [2020-05-17 10:55:39,333: INFO/ForkPoolWorker-3] Task swh.loader.package.nixguix.tasks.LoadNixguix[7982729e-96af-4700-8ae0-afa499a10830] succeeded in 2393.330544631928s: {'status': 'eventful', 'snapshot_id': 'a990925694c53453311030c652570901848b8a1d'}

[1] which will be improved upon later

[2] This is the files declaring the artifacts for the nixguix_loader to ingest.

For people witch access to the staging infra, the latest snapshot is browsable [1]

[1] https://webapp.internal.staging.swh.network/swh:1:snp:a990925694c53453311030c652570901848b8a1d;origin=https://nix-community.github.io/nixpkgs-swh/sources-unstable.json

The main loader is now deployed.
Another task got opened to keep the references on the possible improvments [1]

[1] T2879