MDL-39602 cache: Move the includes for default stores

Putting them inside the function get_default_stores ensures
that they will be included from anywhere that calls get_default_stores.

cache/locallib.php

@@ -393,15 +393,9 @@ public function delete_store_instance($name) {
      *     Returns a configuration array if it could not be saved. This is a bad situation. Check your error logs.
      */
     public static function create_default_configuration($forcesave = false) {
-        global $CFG;
-
         // HACK ALERT.
         // We probably need to come up with a better way to create the default stores, or at least ensure 100% that the
         // default store plugins are protected from deletion.
-        require_once($CFG->dirroot.'/cache/stores/file/lib.php');
-        require_once($CFG->dirroot.'/cache/stores/session/lib.php');
-        require_once($CFG->dirroot.'/cache/stores/static/lib.php');
-
         $writer = new self;
         $writer->configstores = self::get_default_stores();
         $writer->configdefinitions = self::locate_definitions();
@@ -448,6 +442,12 @@ public static function create_default_configuration($forcesave = false) {
      * @return array
      */
     protected static function get_default_stores() {
+        global $CFG;
+
+        require_once($CFG->dirroot.'/cache/stores/file/lib.php');
+        require_once($CFG->dirroot.'/cache/stores/session/lib.php');
+        require_once($CFG->dirroot.'/cache/stores/static/lib.php');
+
         return array(
             'default_application' => array(
                 'name' => 'default_application',