core: Deal transparently with no 'visit_type' attribute in loaders
Currently, the current code fails multiple loaders (not exhaustive):
- https://jenkins.softwareheritage.org/job/DLDDIR/job/tests/321/console
- https://jenkins.softwareheritage.org/job/DDEP/job/tests/414/console
This is a transition and this should be removed when we are done with the migration.
Test Plan
tox
Migrated from D1945 (view on Phabricator)