report source path and let front-end resolve implicit master location (e.g. URL);

--- a/src/Pure/Thy/thy_load.ML	Fri Aug 24 20:41:47 2012 +0200
+++ b/src/Pure/Thy/thy_load.ML	Fri Aug 24 20:47:33 2012 +0200
@@ -68,8 +68,8 @@
   let
     fun make_file file =
       let
+        val _ = Position.report pos (Isabelle_Markup.path (Path.implode file));
         val full_path = check_file dir file;
-        val _ = Position.report pos (Isabelle_Markup.path (Path.implode full_path));
       in {src_path = file, text = File.read full_path, pos = Path.position full_path} end;
     val paths =
       (case Keyword.command_files cmd of