external-lib-deps --missing could suggest installing MDX for MDX stanzas

When attempting dune build in a repository using MDX stanzas (with no external libraries initially available), the user gets an experience like the following:

• run dune build;
• fail due to libraries not available. run dune external-lib-deps --missing @@default;
• run the suggested opam install ... command;
• run dune build again;
• fail due to missing mdx dependency for the MDX stanza, with a nice error message:

File "tests/dune", line 1, characters 0-45:
1 | (mdx
2 |  (files run.t)
3 |  (packages foo))
Error: Program ocaml-mdx not found in the tree or in PATH
 (context: default)
Hint: opam install mdx

This could be improved slightly by considering mdx as part of dune external-lib-deps @@foo if @foo will trigger an MDX stanza.

Reproduce the reported workflow with the MDX stanza shown in tests/dune, starting with dune build and then dune external-lib-deps --missing @@default. Trace the external-lib-deps handling for targets that trigger an MDX stanza. Done means the generated missing-dependencies installation command includes mdx before the subsequent build.