config: move Config namespace to AutoloadConfig

We can't customize `Config` namespace name.

Author: kenjis

app/Config/Autoload.php

@@ -46,7 +46,6 @@ class Autoload extends AutoloadConfig
      */
     public $psr4 = [
         APP_NAMESPACE => APPPATH, // For custom app namespace
-        'Config'      => APPPATH . 'Config',
     ];
 
     /**

system/Config/AutoloadConfig.php

@@ -93,6 +93,7 @@ class AutoloadConfig
     protected $corePsr4 = [
         'CodeIgniter' => SYSTEMPATH,
         'App'         => APPPATH, // To ensure filters, etc still found,
+        'Config'      => APPPATH . 'Config',
     ];
 
     /**