getVersion: first try drv.version before parsing drv.name

lib/strings.nix

@@ -372,7 +372,12 @@ rec {
        getVersion pkgs.youtube-dl
        => "2016.01.01"
   */
-  getVersion = x: (builtins.parseDrvName (x.name or x)).version;
+  getVersion = x:
+   let
+     parse = drv: (builtins.parseDrvName drv).version;
+   in if isString x
+      then parse x
+      else x.version or (parse x.name);
 
   /* Extract name with version from URL. Ask for separator which is
      supposed to start extension.