Make the opam loader write extrinsic metadata

https://docs.softwareheritage.org/devel/swh-loader-core/package-loader-tutorial.html#loading-metadata

---

Migrated from T3636 (view on Phabricator)

added Extrinsic metadata Opam priority:Normal labels

marked this issue as related to #3638 (closed)

(this task is a dependency of #3638 (closed), because author != committer in revisions created by opam, and releases don't have a "committer" field, so switching to releases would lose this data)

marked this issue as related to swh/infra/sysadm-environment#3722 (closed)

assigned to @vlorentz

closed