Fix #3741 Handle global Drush9 edge case (#3746)

* Handle global Drush9 edge case - #3741.

* Add docs link.

Author: weitzman

src/Config/Environment.php

@@ -59,6 +59,10 @@ public function loadSiteAutoloader($root)
         }
 
         $this->siteLoader = require $autloadFilePath;
+        if ($this->siteLoader === false) {
+            // Nothing more to do. See https://github.com/drush-ops/drush/issues/3741.
+            return $this->loader;
+        }
         if ($this->siteLoader === true) {
             // The autoloader was already required. Assume that Drush and Drupal share an autoloader per
             // "Point autoload.php to the proper vendor directory" - https://www.drupal.org/node/2404989

sut/autoload.php

@@ -11,4 +11,5 @@
  * @see core/modules/statistics/statistics.php
  */
 
-return require __DIR__ . '/../vendor/autoload.php';
+// Use include instead of return because See https://github.com/drush-ops/drush/issues/3741
+return include __DIR__ . '/../vendor/autoload.php';