Change deprecated modules in a separate directory

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

src/module.ml

@@ -176,6 +176,12 @@ let deprecate t =
           | Some intf, _ -> intf.path
           | None, None -> assert false
         in
+        let path =
+          Path.L.relative (Path.parent_exn path)
+            [ ".deprecated"
+            ; Path.basename path
+            ]
+        in
         let (base, _) = Path.split_extension path in
         { syntax = OCaml
         ; path = Path.extend_basename base ~suffix:".ml-gen"

src/stdune/path.ml

@@ -943,3 +943,8 @@ module Internal = struct
     | In_source_tree l -> Local l
     | External l -> External l
 end
+
+module L = struct
+  (* TODO more efficient implementation *)
+  let relative t = List.fold_left ~init:t ~f:relative
+end

src/stdune/path.mli

@@ -169,3 +169,7 @@ val set_root : External.t -> unit
 module Internal : sig
   val raw_kind : t -> Kind.t
 end
+
+module L : sig
+  val relative : t -> string list -> t
+end