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

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Migrated from D1945 (view on Phabricator)