Rework loader instantiation logic according to loader core api
The new loader.core now passes all configuration through the constructor. So no more default configuration only constructor parameters.
Note that this also updated some docstrings and type along the way.
impacts:
- swh-web!487 (closed): swh.web
- swh-vault!80 (closed): swh.vault
Related to swh-core#1410
Test Plan
tox (failing until swh.loader.core > 0.17 is released)
Migrated from D5077 (view on Phabricator)