Add modules_without_implementation example to wrapped transition mode

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/module.ml

@@ -166,11 +166,16 @@ let to_sexp { name; impl; intf; obj_name ; pp } =
 
 let deprecate t =
   { t with
-    intf = None
-  ; impl =
+    impl =
       Some (
-        let impl = Option.value_exn t.impl in
-        let (base, _) = Path.split_extension impl.path in
+        let path =
+          match t.intf, t.impl with
+          | Some _, Some impl
+          | None, Some impl -> impl.path
+          | Some intf, _ -> intf.path
+          | None, None -> assert false
+        in
+        let (base, _) = Path.split_extension path in
         { syntax = OCaml
         ; path = Path.extend_basename base ~suffix:".ml-gen"
         }

test/blackbox-tests/test-cases/wrapped-transition/fooexe.ml

@@ -2,3 +2,6 @@ Mylib.Bar.run ();;
 Mylib.Foo.run ();;
 Bar.run ();;
 Foo.run ();;
+
+module Y : Mylib.Intf_only.S = struct end
+module X : Intf_only.S = struct end

test/blackbox-tests/test-cases/wrapped-transition/lib/dune

@@ -1,3 +1,4 @@
 (library
  (name mylib)
+ (modules_without_implementation intf_only)
  (wrapped (transition "Will be removed past 2020-20-20")))

test/blackbox-tests/test-cases/wrapped-transition/lib/intf_only.mli

@@ -0,0 +1 @@
+module type S = sig end