Fix MissingDirPlugin_DrupalExtensionPrefix, for module names with underscores.

lib/MissingDirPlugin/DrupalExtensionPrefix.php

@@ -4,6 +4,7 @@ class xautoload_MissingDirPlugin_DrupalExtensionPrefix extends xautoload_Missing
 
   function alternativeDir($path_fragment) {
     $extension = substr($path_fragment, 0, -1);
+    $extension = str_replace(array('/', DIRECTORY_SEPARATOR), '_', $extension);
     if ($filepath = drupal_get_filename($this->type, $extension)) {
       if ($this->shallow) {
         return dirname($filepath) . '/lib/';