ENGCOM-3295: Autoload for vendor root folders #18849

 - Merge Pull Request #18849 from Rus0/magento2:m2-17753
 - Merged commits:
   1. 2054fab

Author: magento-engcom-team

app/autoload.php

@@ -28,6 +28,9 @@
 /* 'composer install' validation */
 if (file_exists($vendorAutoload)) {
     $composerAutoloader = include $vendorAutoload;
+} else if (file_exists("{$vendorDir}/autoload.php")) {
+	$vendorAutoload = "{$vendorDir}/autoload.php";
+	$composerAutoloader = include $vendorAutoload;
 } else {
     throw new \Exception(
         'Vendor autoload is not found. Please run \'composer install\' under application root directory.'