MDL-28049 conditional include so xml question format can work for both lesson and 3rd party qtypes.

This is a regression caused by MDL-27956.

question/format/xml/format.php

@@ -27,6 +27,14 @@
 defined('MOODLE_INTERNAL') || die();
 
 require_once($CFG->libdir . '/xmlize.php');
+if (!class_exists('qformat_default')) {
+    // This is ugly, but this class is also (ab)used by mod/lesson, which defines
+    // a different base class in mod/lesson/format.php. Thefore, we can only
+    // include the proper base class conditionally like this. (We have to include
+    // the base class like this, otherwise it breaks third-party question types.)
+    // This may be reviewd, and a better fix found one day.
+    require_once($CFG->dirroot . '/question/format.php');
+}
 
 
 /**

question/format/xml/simpletest/testxmlformat.php

@@ -27,7 +27,6 @@
 defined('MOODLE_INTERNAL') || die();
 
 require_once($CFG->libdir . '/questionlib.php');
-require_once($CFG->dirroot . '/question/format.php');
 require_once($CFG->dirroot . '/question/format/xml/format.php');