tox: split tests by 'extra' subpackage
so each 'extra' subpackage is tested in a minimal venv (with minimal dependencies installed).
This requires to remove extras-needed depencies from the main requirements.txt
Warning: this may break swh packages that depends on swh.core which have not been updated to swh.core 0.0.60.
Test Plan
tox should still be happy
Migrated from D1444 (view on Phabricator)