Revert "Check for the full stack based path first."

This reverts commit 80a1987a5f7f3dc28424256dbb2741ed64037c0f. By
checking the full-stack path first, we run the risk of loading the
incorrect cakephp-plugins.php file as we're reaching outside of the
application root directory.

Refs #8055

src/Core/Plugin.php

@@ -185,9 +185,9 @@ class Plugin
         if (Configure::check('plugins')) {
             return;
         }
-        $vendorFile = dirname(dirname(dirname(dirname(__DIR__)))) . DS . 'cakephp-plugins.php';
+        $vendorFile = dirname(dirname(__DIR__)) . DS . 'cakephp-plugins.php';
         if (!file_exists($vendorFile)) {
-            $vendorFile = dirname(dirname(__DIR__)) . DS . 'cakephp-plugins.php';
+            $vendorFile = dirname(dirname(dirname(dirname(__DIR__)))) . DS . 'cakephp-plugins.php';
             if (!file_exists($vendorFile)) {
                 Configure::write(['plugins' => []]);
                 return;