Move 3rd-party libs to a dedicated directory

[MAILPOET-2545]

lib/Util/pQuery/pQuery.php

@@ -3,6 +3,7 @@
 namespace MailPoet\Util\pQuery;
 
 // extend pQuery class to use UTF-8 encoding when getting elements' inner/outer text
+// phpcs:ignore Squiz.Classes.ValidClassName
 class pQuery extends \pQuery {
   public static function parseStr($html) {
     $parser = new Html5Parser($html);