[Backport] Declare module namespace before template path in Magento_Theme, Magento_Newsletter and Magento_Tax

[mageprince]

Original Pull Request

#16699
Relates PR
#16515

Description

When we override any core block to any custom module, It throws an error because it tries to find template file in the custom module if we do not declare module namespace before template path.
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: 'form/newsletter.phtml' in module: 'Vendor_Module'

Fixed Issues (if relevant)

Declare module namespace before template path name.
For example: protected $_template = 'Magento_Customer::form/newsletter.phtml';

Manual testing scenarios

Override any block file which content static template file path

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)

[magento-engcom-team]

Hi @mageprince. Thank you for your contribution.
We will aim to release these changes as part of 2.1.15.
Please check the release notes for final confirmation.