Depext not building at all (and no hint) when no `libraries` field

[ElectreAAS]

Expected Behavior

On a simple project with a singular dependency on conf-foobar:

  $ cat > dune-project << EOF
  > (lang dune 3.20)
  > (package
  >  (name foo)
  >  (depends conf-foobar))
  > EOF

  $ cat > src/dune << EOF
  > (executable
  >  (name foo)
  >  (public_name foo))
  > EOF
No 'libraries' field here ^^

  $ cat > src/foo.ml << EOF
  > let _exit_code = Sys.command "foobar some_arg" in
  > ()
  > EOF

Assuming conf-foobar actually exists, one would expect either dune pkg lock or dune build (or both) to say something about a missing system dependency, but they don't (see reproduction PR #12857).

Actual Behavior

After the setup above, dune pkg lock correctly says "Solution for dune.lock [...] conf-foobar.0.0.1", dune build doesn't say anything, and only when we dune exec do we get "sh: line 1: foobar: command not found" and no hint!

Reproduction

• PR with a reproducing test: Repro test: no hint about missing depext #12857