Ensure missing targets which depend on MightNotUpdate targets are created

I think this commit should solve the issue which I opened regarding the MightNotUpdate dependencies causing their parents to not be updated.  It feels more like a work-around than a fix though.  Can you see any issues with it or a better way to do this?

Currently the complicated checks in the case for MightNotUpdate children end up
forcing the parent to a stable fate when the child has not updated, even when
the parent is bound to a missing file.

This work-around avoids the special handling for MightNotUpdate children when
the parent is bound to a missing file.