* Add --latest to mirror only the newest version
An unversioned PURL mirrors every version the upstream lists. With
--latest, or "latest": true in a mirror API job, only the newest is
mirrored: the highest by the ecosystem's version ordering, without
yanked or retracted versions and, unless there is nothing else,
pre-releases.
* Pick the latest version with registries.FetchLatestVersion
--latest now uses the shared policy from git-pkgs/registries: the version
the registry advertises as latest where it has one, otherwise the newest
active version by publish time. Deprecated versions are left out too, and
newestVersion and the direct vers dependency go away.
