Move XSLXWriter to MailPoetVendor namespace

[MAILPOET-2545]

lib-3rd-party/XLSXWriter.php

@@ -1,6 +1,6 @@
 <?php
 
-namespace MailPoet\Util;
+namespace MailPoetVendor;
 
 /*
  * @license MIT License

lib/Subscribers/ImportExport/Export/Export.php

@@ -6,8 +6,8 @@ use MailPoet\Config\Env;
 use MailPoet\Models\CustomField;
 use MailPoet\Subscribers\ImportExport\ImportExportFactory;
 use MailPoet\Util\Security;
-use MailPoet\Util\XLSXWriter;
 use MailPoet\WP\Functions as WPFunctions;
+use MailPoetVendor\XLSXWriter;
 
 use function MailPoetVendor\array_column;
 

tests/unit/Util/XlsxWriterTest.php

@@ -8,7 +8,7 @@ class XlsxWriterTest extends \MailPoetUnitTest {
   }
 
   public function testItCanBeCreated() {
-    $writer = new \MailPoet\Util\XLSXWriter();
+    $writer = new \MailPoetVendor\XLSXWriter();
   }
 
   public function _after() {