MDL-49329 admin: Improve the plugin package validator

Previously, the validator was used for installation of the new plugins
only. We now validate every incoming plugin package. As a result, we
must no longer insists on the target location is empty. Instead, warning
is raised.

lib/classes/plugin_manager.php

@@ -1807,7 +1807,7 @@ class core_plugin_manager {
      * @param string $fullpath
      * @return boolean
      */
-    protected function is_directory_removable($fullpath) {
+    public function is_directory_removable($fullpath) {
 
         if (!is_writable($fullpath)) {
             return false;