Testing composition with a boot library with plugins

  $ dune build
  Warning: Coq Language Versions lower than 0.8 have been deprecated in Dune
  3.8 and will be removed in an upcoming Dune version.
  Hint: To disable this warning, add the following to your dune-project file:
  (warnings (deprecated_coq_lang_lt_08 disabled))
  plugin loaded
  plugin loaded
