Add absolute path to cake/vendors.

When the app + cake are on different paths, `VENDORS` is incorrect, and
the built-in repo/vendors directory is not reachable by the default
paths.

Fixes #2545

lib/Cake/Core/App.php

@@ -861,7 +861,9 @@ protected static function _packageFormat() {
 					'%s' . 'locale' . DS
 				),
 				'Vendor' => array(
-					'%s' . 'Vendor' . DS, VENDORS
+					'%s' . 'Vendor' . DS,
+					dirname(dirname(CAKE)) . DS . 'vendors' . DS,
+					VENDORS
 				),
 				'Plugin' => array(
 					APP . 'Plugin' . DS,