Fix printing internal libs with the command "dune describe external-lib-deps"

[moyodiallo]

When the internal libs are added as dependency by another dune library or dune executable, those are printed.

[esope]

It would really help understanding your fix if you could give an example that first shows the produced output and the expected output, and explains why the expected output should be the right one.
I currently have no idea of what the intended semantics for external-lib-deps should be...

[moyodiallo]

dune describe external-lib-deps is supposed to print only external libraries by dir which contain a dune file.
I simplify the test that I added with more understanding. Feel free to reformulate it.

This PR refers to #6045

[moyodiallo]

This was the print before.

(default
 ((.
   ((a________ required)
    (inter_lib required)))
  (lib ((a________ required)))))

and the right one is

(default
 ((. ((a________ required)))
  (lib ((a________ required)))))

This is because inter_lib is an internal lib.

Refer to the tests that I added.

[moyodiallo]

Is it possible to have this PR for dune 3.6 ?
There's a need for ocurrent/opam-dune-lint#46.

@emillon

[emillon]

Thanks!