Fix various compatibility issues with PHP 8

libs/swiftmailer/dependency_maps/message_deps.php

@@ -0,0 +1,9 @@
+<?php
+
+Swift_DependencyContainer::getInstance()
+    ->register('message.message')
+    ->asNewInstanceOf('Swift_Message')
+
+    ->register('message.mimepart')
+    ->asNewInstanceOf('Swift_MimePart')
+;