MDL-47206 core: Allow tool plugins to run tasks

This issue is a part of the MDL-47194 Task.
This issue is a part of the MDL-39707 Epic.

lib/classes/plugininfo/tool.php

@@ -36,6 +36,15 @@ class tool extends base {
         return true;
     }
 
+    /**
+     * Tools cannot be disabled.
+     *
+     * @return boolean
+     */
+    public function is_enabled() {
+        return true;
+    }
+
     /**
      * Return URL used for management of plugins of this type.
      * @return moodle_url