Make sure that Alias0.dir always goes through dir check

the directory must always be inside the build dir, so we make sure that any way
to create the record validates this invariant

src/build_system.ml

@@ -212,7 +212,27 @@ module File_spec = struct
 end
 
 module Alias0 = struct
-  type t = { dir : Path.t; name : string }
+  module T : sig
+    type t = private
+      { dir : Path.t
+      ; name : string
+      }
+    val make : string -> dir:Path.t -> t
+  end = struct
+    type t =
+      { dir : Path.t
+      ; name : string
+      }
+
+    let make name ~dir =
+      if not (Path.is_in_build_dir dir) || String.contains name '/' then
+        Exn.code_error "Alias0.make: Invalid alias"
+          [ "name", Sexp.To_sexp.string name
+          ; "dir", Path.sexp_of_t dir
+          ];
+      { dir; name }
+  end
+  include T
 
   let pp fmt t = Path.pp fmt (Path.relative t.dir t.name)
 
@@ -222,24 +242,13 @@ module Alias0 = struct
     if not (Path.is_in_build_dir path) then
       die "Invalid alias!\nTried to reference alias %S"
         (Path.to_string_maybe_quoted path);
-    { dir  = Path.parent path
-    ; name = Path.basename path
-    }
+    make ~dir:(Path.parent path) (Path.basename path)
 
   let name t = t.name
   let dir  t = t.dir
 
   let fully_qualified_name t = Path.relative t.dir t.name
 
-  let make name ~dir =
-    assert (not (String.contains name '/'));
-    if not (Path.is_in_build_dir dir) then
-      Exn.code_error "Alias0.make: Invalid alias"
-        [ "name", Sexp.To_sexp.string name
-        ; "dir", Path.sexp_of_t dir
-        ];
-    { dir; name }
-
   let stamp_file t =
     Path.relative (Path.insert_after_build_dir_exn t.dir ".aliases") (t.name ^ suffix)