Use different method to detect relative paths

see edolstra#18

default.nix

@@ -40,7 +40,7 @@ let
       })
     else if info.type == "path" then
       { outPath = builtins.path {
-          path = if builtins.substring 0 1 info.path == "."
+          path = if builtins.substring 0 1 info.path != "/"
             then src + ("/" + info.path)
             else info.path; };
         narHash = info.narHash;