Related to T3922

Related to T2610

Defining the pytest-plugin though the pytest-plugin [1] makes it loaded by default.
This creates loading issues on modules depending on storage but not on
the pytest plugin storage exposes. It was explained in the doc and I did not realize [2]

Instead we'll explicitely define to modules depending on the pytest plugins in
their root conftest [3]:

```
pytest_plugins = [ "swh.storage.pytest_plugin" ]
```

[1] https://docs.pytest.org/en/stable/writing_plugins.html#setuptools-entry-points

[2] https://docs.pytest.org/en/stable/writing_plugins.html#plugin-discovery-order-at-tool-startup

[3] https://docs.pytest.org/en/stable/writing_plugins.html#requiring-loading-plugins-in-a-test-module-or-conftest-file

Related to T2484

This will allow loaders to reuse those dedicated fixtures within their code
base without having to import the swh.storage.tests.conftest module.

Related to T2484

Related to T2105

Related to T2367

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

we do add this 'storage' group even if it contains only one subcommand
(serve) for UI consistency:

  swh storage rpc-serve

We do keep the main swh-storage console_script entry point as is for
bw compat, however.

related to T1242

This package is inaugurated by the distribution schemata

As for python3-swh.storage.archiver, the runtime dependency on
python3-swh.core is the implicit one from python3-swh.storage.

The same note about splitting repository and adding explicit dependency
applies.

Related T557

Closes T460

Update the setup.py file in order to add the new created directory 'swh/storage/checker'

Closes T404