Misses mdx dependencies entailed by (using mdx l.m) stanzas

[shonfeder]

Dune has built-in support for mdx, but the stanzas don't require the usual libraries clause. As a result of this edge case, I assume, opam-dune-lint doesn't detect this dependency.

To reproduce, create a project that uses the mdx stanza, and run ocaml-opam-lint on it.

[talex5]

opam-dune-lint uses dune external-lib-deps @runtest to get the dependencies. If the project's tests depend on mdx but dune doesn't list it there, then that sounds like a bug in dune.