loader.core: check_snapshot: Allow to check using both Snapshot and dict objects
So we can smoothly transition to stop using snapshot as dict (as input).
Next step is making the check_snapshot check first the existence of objects targetted from the snapshot (first level, release, revision and alias consistency) [1]
(And then check the remaining parts)
Related to T2483
- [1] D3478
Test Plan
tox
Migrated from D3473 (view on Phabricator)