The maven loader loads jar and zip files as Maven artefacts into the software heritage archive.

Note:
Supersedes D6158 and addresses the review done in that diff.

Related to T1724

Related to T2610

This does not expose the pytest-plugin through the setuptools entry as this can
be problematic. Instead that relies on the specific modules (loader-*)
declaring they need that dependency in their root conftest.

Related to D3452
Related to D3430
Related to T2484

- blackify all the python files,
- enable black in pre-commit,
- add a black tox environment.

- the module functional is renamed to nixguix
- the class FunctionalLoader to NixGuixLoader

This loader is in charge of loading sources of Nix/Guix
evaluations. These sources are listed in a JSON file which is ingested
by this loader.

swh-storage >= v0.0.174 requires swh-model objects in other proxies,
so the validation proxy must be before the other ones.

Required by swh-storage >= v0.0.172.

Related T2026

Related to T2134

Related to T2134

Related to T2134

Related to T2134

Related to T2134

Which demonstrates that the origin type for now is still mandatory. So right
now, the current loading tests fail.

Contrary to other package loader, all artifacts (for a version) are ingested as
one revision (if that makes any sense).

Caveat: This does not implement a release as the initial loader (yet?)

Related T2025
Related D2133
Related D2134

Related D2084
Related D2085

Prior to this commit, the DATADIR was computed. It turns out most of the time,
this can be deduced from the runtime execution.

We also unified the "datadir" path/to/data.

Tests green

for a given url

For such given urls, this returns a 404 wherever if the representation file
exists or not.

on 'https://'.
Refactor existing tests to use this fixture.

This allows to make test execution not depend on the SWH_CONFIG_FILENAME
environment variable, so it's easier to run them using pytest directly
(rather than tox).