Restore part of the autoloader structure; it broke PHP 8.x support

lib/_autoload.php

+<?php
+
+/**
+ * This file is a backwards compatible autoloader for SimpleSAMLphp.
+ * Loads the Composer autoloader.
+ *
+ * @package SimpleSAMLphp
+ */
+
+declare(strict_types=1);
+
+// SSP is loaded as a separate project
+if (file_exists(dirname(dirname(__FILE__)) . '/vendor/autoload.php')) {
+    require_once dirname(dirname(__FILE__)) . '/vendor/autoload.php';
+} else {
+    // SSP is loaded as a library
+    if (file_exists(dirname(dirname(__FILE__)) . '/../../autoload.php')) {
+        require_once dirname(dirname(__FILE__)) . '/../../autoload.php';
+    } else {
+        throw new Exception('Unable to load Composer autoloader');
+    }
+}

www/_include.php

 <?php
 
+// initialize the autoloader
+require_once(dirname(dirname(__FILE__)) . '/lib/_autoload.php');
+
 use SAML2\Compat\ContainerSingleton;
 use SimpleSAML\Compat\SspContainer;
 use SimpleSAML\Configuration;