[Move.lock] Avoid package resolution if manifest unchanged

[amnn]

Depends on #8342

Once lock files are enabled for Sui, it will be possible to rely on them, instead of the manifest, as a complete picture of a package's transitive dependency graph. This means that if a package's manifest has not been modified since a lock file was last generated from it, it does not need to be re-generated.

Generating a lock file from a manifest can be a costly process, because it requires fetching each dependency in turn, and discovering from that dependency's manifest what further packages to fetch, and so on, recursively, which is difficult to parallelise.

If this step can be avoided, there are further gains to be had, by parallelising transitive dependency fetching.