Fix OriginInfo: use a list instead of a set for visit_types

This matches what swh-search returns and expects. This could make
test_lookup_origin() (in test_archive.py) fail when the bad luck (?) made
hypothesis generate a test set where swh-search's in-memory backend
tries to merge the visit_types with an existing one (thus trying to add
a pair set objects).

Adapt the definition of OriginInfo (more or less the local version of
swh-search's OriginDict) accordingly.