test(package-deps): snapshot the missing Melange closure

test/blackbox-tests/test-cases/package-materialization/melange-closure.t

@@ -0,0 +1,75 @@
+A scoped package layout retains a library's Melange-only requirements in its
+`dune-package` file, so it must materialize the corresponding Melange closure.
+
+  $ make_dune_project 3.24
+  $ cat >>dune-project <<'EOF'
+  > (using melange 0.1)
+  > (package (name melange-root))
+  > (package (name melange-support))
+  > EOF
+
+  $ mkdir root support consumer
+  $ cat >support/dune <<'EOF'
+  > (library
+  >  (name melange_support)
+  >  (public_name melange-support)
+  >  (modes melange))
+  > EOF
+  $ echo 'let value = 42' >support/melange_support.ml
+
+  $ cat >root/dune <<'EOF'
+  > (library
+  >  (name melange_root)
+  >  (public_name melange-root)
+  >  (modes melange)
+  >  (melange.libraries melange-support))
+  > EOF
+  $ echo 'let value = Melange_support.value' >root/melange_root.ml
+
+  $ cat >consumer/dune-project <<'EOF'
+  > (lang dune 3.24)
+  > (using melange 0.1)
+  > EOF
+  $ cat >consumer/dune <<'EOF'
+  > (melange.emit
+  >  (target out)
+  >  (emit_stdlib false)
+  >  (modules main)
+  >  (libraries melange-root))
+  > EOF
+  $ echo 'let () = Js.log Melange_root.value' >consumer/main.ml
+
+  $ cat >dune <<'EOF'
+  > (rule
+  >  (target result)
+  >  (deps
+  >   (package melange-root)
+  >   (source_tree consumer))
+  >  (action
+  >   (with-stdout-to %{target}
+  >    (chdir consumer (run %{bin:dune} build @melange)))))
+  > EOF
+
+The OCaml-only support closure leaves the serialized `melange_requires` edge
+pointing at a library that is absent from the layout.
+
+  $ dune build result 2>err
+  [1]
+  $ censor <err
+  File "$PWD/_build/install/default/.packages/$DIGEST/lib/melange-root/dune-package", line 15, characters 19-34:
+  15 |  (melange_requires melange-support)
+                          ^^^^^^^^^^^^^^^
+  Error: Library "melange-support" not found.
+  -> required by library "melange-root" in
+     $PWD/_build/install/default/.packages/$DIGEST/lib/melange-root
+  -> required by melange target out
+  -> required by alias melange
+  File "$PWD/_build/install/default/.packages/$DIGEST/lib/melange-root/dune-package", line 15, characters 19-34:
+  15 |  (melange_requires melange-support)
+                          ^^^^^^^^^^^^^^^
+  Error: Library "melange-support" not found.
+  -> required by melange target out
+  -> required by library "melange-root" in
+     $PWD/_build/install/default/.packages/$DIGEST/lib/melange-root
+  -> required by _build/default/out/main.js
+  -> required by alias melange