package.loader: Fix serialization issue
And aligns behavior in between loader packages tests:
- package.debian: Align origin_visit type to 'deb' as in production
- package.loader: Ensure the origin visit type is set appropriately
- package.loader: Check snapshot_id is set as returned value
Just to make sure ci is green
Test Plan
tox
Migrated from D2270 (view on Phabricator)