package loader: Add missing filtering on redundant built revisions

To avoid this kind of issues.

Detected by the opam loader (staging) [2] but this could happen on other package loader implementations.

• [1] https://sentry.softwareheritage.org/share/issue/5eea922342c94de5ad82c9d303c77bda/

• [2] swh/infra/sysadm-environment#3438 (closed)

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Migrated from T3469 (view on Phabricator)