test(melange): demonstrate melange.emit doesn't respect -p

Signed-off-by: Antonio Nuno Monteiro <anmonteiro@gmail.com>

test/blackbox-tests/test-cases/melange/melange-emit-package.t

@@ -0,0 +1,43 @@
+
+  $ mkdir lib test ppx
+  $ cat > dune-project <<EOF
+  > (lang dune 3.8)
+  > (package (name my-ppx))
+  > (package (name mel-foo))
+  > (using melange 0.1)
+  > EOF
+
+  $ cat > ppx/dune <<EOF
+  > (library
+  >  (name my_ppx)
+  >  (public_name my-ppx))
+  > EOF
+  $ touch ppx/my_ppx.ml
+
+  $ cat > lib/dune <<EOF
+  > (library
+  >  (public_name mel-foo)
+  >  (name mel_foo)
+  >  (modes melange)
+  >  (preprocess (pps melange.ppx)))
+  > EOF
+  $ cat > lib/mel_foo.ml <<EOF
+  > let x = "lib"
+  > EOF
+
+  $ cat > test/dune <<EOF
+  > (melange.emit
+  >  (package mel-foo)
+  >  (target js-out)
+  >  (libraries mel-foo))
+  > EOF
+
+`melange.emit` is attached to the package `mel-foo`, so it shouldn't be built
+when building the other library
+
+  $ dune build -p my-ppx
+  File "test/dune", line 4, characters 12-19:
+  4 |  (libraries mel-foo))
+                  ^^^^^^^
+  Error: Library "mel-foo" not found.
+  [1]