Fix error message about too many opam files

CHANGES.md

@@ -1,3 +1,9 @@
+1.0.0 (coming soon)
+-------------------
+
+- Fix the error message when there are more than one `<package>.opam`
+  file for a given pacakge
+
 1.0+beta10 (08/06/2017)
 -----------------------
 

src/jbuild_load.ml

@@ -210,7 +210,7 @@ let load ?(extra_ignored_subtrees=Path.Set.empty) () =
           name
           (String.concat ~sep:"\n"
              (List.map pkgs ~f:(fun pkg ->
-                sprintf "- %s.opam" (Path.to_string pkg.Package.path)))))
+                sprintf "- %s" (Path.to_string (Package.opam_file pkg))))))
   in
   let scopes =
     String_map.values packages

src/package.ml

@@ -3,3 +3,5 @@ type t =
   ; path                   : Path.t
   ; version_from_opam_file : string option
   }
+
+let opam_file t = Path.relative t.path (t.name ^ ".opam")

src/package.mli

@@ -0,0 +1,9 @@
+(** Information about a package defined in the workspace *)
+
+type t =
+  { name                   : string
+  ; path                   : Path.t
+  ; version_from_opam_file : string option
+  }
+
+val opam_file : t -> Path.t