FR: Detect manifest changes and automatically run lock

[thesayyn]

Currently, the lock generation is manual, and sometimes forgotten after manifest changes, it would be nice to detect manifest changes. and trigger lock generation and update the lockfile in place.

[JonathanPerry651]

A thought - If/when we do this, could we have a lockfile mode similar to bzlmod, where you can make it fail if the manifest has changes rather than auto-updating? Otherwise we can end up with spurious diffs being generated into source in CI runs, for example.

Thank you