MDL-34401 Replace ad-hoc plugin request caches with proper application caches

Data cached in these caches change only at well defined places (during
need for upgrade checks, at the plugin management screen etc). So it
makes sense to use proper application caches instead of request caches.

This saves couple of database queries at almost every page in Moodle.

admin/modules.php

@@ -29,6 +29,9 @@
     $stractivitymodule = get_string("activitymodule");
     $strshowmodulecourse = get_string('showmodulecourse');
 
+    // Purge all caches related to activity modules administration.
+    cache::make('core', 'plugininfo_mod')->purge();
+
 /// If data submitted, then process and store.
 
     if (!empty($hide) and confirm_sesskey()) {