Twig templates are not being picked up from the modules

Problem/Motivation

When defining a twig template inside a module it's not being picked up.

It only picks up when we define template in the theme otherwise only picks up swiftmailer.html.twig from swiftmailer/templates/swiftmailer.html.twig

Steps to reproduce

- Create a custom module call 'test_emails'
- Add a hook_mail
- Setup mailhog for mail catching
- Enable twig debugging in developement.services.yml as per https://www.drupal.org/docs/theming-drupal/twig-in-drupal/debugging-twig...
- Set a theme in mailsystem settings page (admin/config/system/mailsystem)
- Add a template in test_emails module i.e. swiftmailer--test-module.html.twig
- Send an email
- The email will not pickup the template and will use swiftmailer.html.twig

Proposed resolution

Discussion and Directions

Remaining tasks

Create a PR

User interface changes

None

API changes

Not known at this stage

Data model changes

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