loader.package.deposit: Drop swh.deposit.client requirement
This unifies with the other package loader implementations. Also, this should fix the current debian build failure [1] (which is the main goal here).
The real fix would be to rework properly the deposit packages/modules but i don't feel like it's a tractable one for the current week.
Test Plan
tox
Migrated from D2315 (view on Phabricator)