entity_get_display() and entity_get_form_display() moved to EntityDisplayRepositoryInterface

entity_get_display() and entity_get_form_display() have been deprecated since before Drupal 8 was released. However, no replacements were ever developed for them despite their usefulness, which has forced core and contrib to rely on them.

#2367933: Move entity_get_(form_)display() to the entity display repository moves them into EntityDisplayRepository, and they are both defined as methods of EntityDisplayRepositoryInterface (implemented by the entity_display.repository service and the deprecated entity.manager service for backwards compatibility). Both functions are still around, and both are still deprecated, but they are now wrappers around the entity_display.repository service.

Before:

entity_get_display('node', 'page', 'default');
entity_get_display('node', 'article', 'teaser');
entity_get_form_display('node', 'page', 'default');

After:

\Drupal::service('entity_display.repository')->getViewDisplay('node', 'page');
\Drupal::service('entity_display.repository')->getViewDisplay('node', 'article', 'teaser');
\Drupal::service('entity_display.repository')->getFormDisplay('node', 'page');