Use diffprereqs files as dependencies, not FORCE

“`diffprereqs`” files are updated whenever the prereqs change, as part
of dependency expansion (and so, obviously, before dependency
comparison). So depending on them is effectively the same as depending
on FORCE iff the dependency changed. However, this approach continues
working across Make restarts.

Previously, if a transform was removed, but Make restarted due to a
parser change, the removal would be ignored because the prereqs would
be updated on the first run but the transform catalog would not. This
commit resolves this issue.