[Backport] Declare module namespace before template path name(Magento_Sales::order/creditmemo.phtml).

[sanganinamrata]

Original Pull Request

magento#16481

Description

When we override block creditmemo(Magento\Sales\Block\Order\Creditmemo) to my custom module.
It throws an error and finding order/creditmemo.phtml in my custom module.
I tried to extend Block Magento\Sales\Block\Order\Creditmemo from my custom module.

1 exception(s):
Exception #0 (Magento\Framework\Exception\ValidatorException): Invalid template file: 'order/creditmemo.phtml' in module: 'Vendor_Module' block's name: 'sales.order.creditmemo'

Fixed Issues (if relevant)

1. Declare module namespace before template path name.
   protected $_template = 'Magento_Sales::order/creditmemo.phtml';

Manual testing scenarios

1. I override creditmemo block

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)