* Allow the output/expression id to be forced to a certain

value; this potentially dangerous feature enables better 
  sharing for those paths for which the content is known in 
  advance (e.g., because a content hash is given).
* Fast builds: if we can expand all output paths of a derive 
  expression, we don't have to build.

corepkgs/fetchurl/fetchurl.fix

@@ -4,6 +4,7 @@ Function(["url", "md5"],
     , ("url", Var("url"))
     , ("md5", Var("md5"))
     , ("name", BaseName(Var("url")))
+    , ("id", Var("md5"))
     ]
   )
 )