Module implementing custom template suggestions altering an already existing hook.
/**
* Implements hook_theme_suggestions_HOOK_alter().
*/
function MYMODULE_theme_suggestions_HOOK_alter(array &$suggestions, array $variables) {
array_unshift($suggestions, 'ADD_MY_MODULE_SUGGESTION');
}
/**
* Implements hook_theme_registry_alter().
*/
function MYMODULE_theme_registry_alter(&$theme_registry) {
// Adding this modules's templates to the bunch.
// Html.html.twig
$theme_registry['ADD_MY_MODULE_SUGGESTION'] = $theme_registry['html'];
$theme_registry['ADD_MY_MODULE_SUGGESTION']['path'] = drupal_get_path('module', 'MYMODULE') . '/templates';
$theme_registry['ADD_MY_MODULE_SUGGESTION']['template'] = 'MY--TEMPLATE';
}