core.loader: Fix Iterable/List typing issues.
- origin_visit_add returns an iterable, not a sequence
- starting with v0.11.6, _add() endpoints now require a List instead of an Iteralble.
All these will trigger type warning with swh.core > 0.2.0, as it will no longer hide the signature of storage endpoints from mypy
Migrated from D3656 (view on Phabricator)