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.

Related to swh-core#1410

Test Plan

tox (failing until swh.loader.core > 0.17 is released)

Migrated from D5075 (view on Phabricator)