%{cmo}, %{cmi}, etc. are supported, but %{cmt} is not, cf. https://dune.readthedocs.io/en/stable/advanced/variables-artifacts.html#variables-for-artifacts

I want to write a rule that works on cmt files, and I can't.
So currently I add a dependency on the cmo (using %{cmo}) and just cross my fingers and hope that the cmt is nearby and not stale.
This feels dirty and fragile.