test: reproduce #7018 (#7037)

Demonstrate various types of module cycles that aren't cycles if we
separate the module interface and module implementation graphs

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

test/blackbox-tests/test-cases/ocamldep-7018.t

@@ -0,0 +1,78 @@
+This test demonstrate a dependency cycle if we consider nodes as modules, but a
+valid dependency graph if we consider the implementations and interfaces of
+modules as having separate dependencies.
+
+Reproduces #7018
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.7)
+  > EOF
+  $ cat >dune <<EOF
+  > (library
+  >  (wrapped false)
+  >  (name foobar))
+  > EOF
+
+  $ cat >x.mli <<EOF
+  > type t = unit
+  > EOF
+
+  $ cat >y.mli <<EOF
+  > val foo : X.t -> unit
+  > EOF
+  $ cat >y.ml <<EOF
+  > let foo _ = ()
+  > EOF
+
+  $ runtest() {
+  > cat >x.ml <<EOF
+  > type t = unit
+  > let () = Y.foo $1
+  > EOF
+  > dune build
+  > }
+
+First we try to construct X.t directly
+
+  $ runtest "()"
+  Error: dependency cycle between modules in _build/default:
+     X
+  -> X
+  -> required by _build/default/foobar.a
+  -> required by alias all
+  -> required by alias default
+  File "x.ml", line 2, characters 15-17:
+  2 | let () = Y.foo ()
+                     ^^
+  Error: This expression has type t but an expression was expected of type X.t
+         X.t is abstract because no corresponding cmi file was found in path.
+  [1]
+
+Now we use a polymorphic type:
+
+  $ runtest "(assert false)" 
+  Error: dependency cycle between modules in _build/default:
+     X
+  -> X
+  -> required by _build/default/foobar.a
+  -> required by alias all
+  -> required by alias default
+  [1]
+
+Or, we can use another module:
+
+  $ cat > unit.ml <<EOF
+  > let x = ()
+  > EOF
+  $ cat > unit.mli <<EOF
+  > val x : X.t
+  > EOF
+
+  $ runtest "Unit.x"
+  Error: dependency cycle between modules in _build/default:
+     X
+  -> X
+  -> required by _build/default/foobar.a
+  -> required by alias all
+  -> required by alias default
+  [1]