need to expand path in order to resolve imports like "~~/src/Tools/Code_Generator";

--- a/src/Pure/Thy/thy_load.scala	Tue Aug 07 17:08:22 2012 +0200
+++ b/src/Pure/Thy/thy_load.scala	Tue Aug 07 17:46:30 2012 +0200
@@ -39,7 +39,7 @@
   /* file-system operations */
 
   def append(dir: String, source_path: Path): String =
-    (Path.explode(dir) + source_path).implode
+    (Path.explode(dir) + source_path).expand.implode
 
   def read_header(name: Document.Node.Name): Thy_Header =
   {