MDL-42078 multiple uninstall improvements and cleanup

Includes:
* update checker refactored to \core\update\ namespace
* plugininfo classes refactored to \core\plugininfo\ namespace
* plugin_manager renamed to core_plugin_manager
* redirect back to original page after plugin uninstall
* fixed assign subplugin uninstall
* move assign subplugins under the assignment in admin tree
* fixed plugininfo for all question related plugin types
* auth uninstall support
* added missing block dependencies
* added theme uninstall
* subplugin types are following the plugin on plugin overview page
* several performance improvements in plugin manager
* new warnigns when plugininfo are outdated or missing
* multiple fixes and other improvements

admin/cli/install_database.php

@@ -167,9 +167,8 @@ if (!$envstatus) {
 }
 
 // Test plugin dependencies.
-require_once($CFG->libdir . '/pluginlib.php');
 $failed = array();
-if (!plugin_manager::instance()->all_plugins_ok($version, $failed)) {
+if (!core_plugin_manager::instance()->all_plugins_ok($version, $failed)) {
     cli_problem(get_string('pluginscheckfailed', 'admin', array('pluginslist' => implode(', ', array_unique($failed)))));
     cli_error(get_string('pluginschecktodo', 'admin'));
 }