[Forwardport] use "Module_Name::template/path" format instead of using template/path i…

[mzeis]

Original Pull Request

magento#14946
…n Block class to ensure extensibility of the class. not doing so causes invalid template errors after extending the block class via preference

Description

Often third party modules override core block classes. If the source block classes reference a template without specifying module name like "Vendor_Module::template_path" then block render fails with "nvalid template file: " error.

Manual testing scenarios

1. Overide the \Magento\Review\Block\Product\ReviewRenderer Block in your custom module via di preference. Leave your custom class empty.
2. Product details should work.

Contribution checklist

• [ x] Pull request has a meaningful description of its purpose
• [ x] 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)

[mzeis]

Already merged in upstream.