Use swh-storage validation proxy earlier in the pipeline.

swh-storage >= v0.0.174 requires swh-model objects in other proxies, so the validation proxy must be before the other ones.

Other loaders will need the same changes, but I'll commit them directly to master instead of bothering you with diffs.

Test Plan

Expected failure for now, as swh-storage v0.0.174 is not released yet

---

Migrated from D2692 (view on Phabricator)