package.archive: Move archive loader to its own namespace
This is pre-requisite work towards the unified cli.
(Other package loaders have been migrated the same way, i won't open similar diffs as this though, it's time consuming for everyone for little benefit)
Note: This implies a scheduler's task_type migration on their backend name (swh/meta$564)
Related to T2134
Test Plan
tox
Migrated from D2395 (view on Phabricator)