[feature/extension-manager] Remove cron's dependency on the extension manager.

Instead a separate cron provider supplies the manager with tasks from the
extension finder.

PHPBB3-10323

tests/cron/tasks/simple_ready.php

@@ -0,0 +1,8 @@
+<?php
+
+class phpbb_cron_task_core_simple_ready extends phpbb_cron_task_base
+{
+	public function run()
+	{
+	}
+}