v0.38.0 — multi-owner support for the imported-by badge

create-badges gains `importers_own_owners`. The external-importer flag was
already generic (the owner comes from github.repository, never hardcoded), but
single-owner: anyone publishing under both a personal account and an org saw
their own second org flagged external.

Empty keeps the previous behaviour, so no existing caller changes. Matching is
on whole owner segments, so `acme` does not swallow `acme-labs`.