pkg: adding a lockdir should invalidate built artifacts and force a rebuild

[gridbugs]

Dune allows artifacts to be built without a lockdir present, but the resulting artifacts may be different when built with a lockdir present, vs when built without a lockdir present. Currently if a lockdir is created after some artifacts have been built, those artifacts won't be rebuilt by later calls to dune build (unless dune clean is called first).

[rgrinberg]

Could you add a test case for this?