Template suggestions should be based on the template name, not layout id

I had an "INVALID FILE NAME SUGGESTIONS" error in the Twig debug comments because of the missing hook prefix in suggestions.
I came up with a quite similar fix but setting the prefix the following way :

$template_id = $variables['content']['#layout']->getThemeHook()

Will try to get 🐛 Template suggestions does not work Active into 8.x-3.x from 5.0.x