ENGCOM-2208: [Backport] Declare module namespace before template path…

… name(Magento_Sales::order/creditmemo.phtml). #16586

 - Merge Pull Request #16586 from mageprince/magento2:2.1-develop-PR-port-16481
 - Merged commits:
   1. e39ca3e

app/code/Magento/Sales/Block/Order/Creditmemo.php

@@ -17,7 +17,7 @@ class Creditmemo extends \Magento\Sales\Block\Order\Creditmemo\Items
  /**
  * @var string
  */
- protected $_template = 'order/creditmemo.phtml';
+ protected $_template = 'Magento_Sales::order/creditmemo.phtml';
 
  /**
  * @var \Magento\Framework\App\Http\Context