Align event subscribers

Problem/Motivation

Events Subscriber are a mess:

 display_builder_entity_view.event_subscriber:
   class: '\Drupal\display_builder_entity_view\Event\DisplayBuilderEntityViewEventsSubscriber

display_builder_page_layout.page_variant_subscriber:
   class: Drupal\display_builder_page_layout\EventSubscriber\DisplayBuilderPageDisplayVariantSubscriber

display_builder_page_layout.page_layout_event_subscriber:
   class: Drupal\display_builder_page_layout\EventSubscriber\DisplayBuilderPageLayoutEventsSubscriber

 display_builder_views.page_layout_event_subscriber:
   class: Drupal\display_builder_views\EventSubscriber\DisplayBuilderViewsLayoutEventsSubscriber

Proposed resolution

Align them with a consistent naming.

For example:

For subscribing to Drupal\display_builder\Event\DisplayBuilderEvents:

 display_builder_entity_view.display_builder_subscriber:
   class: '\Drupal\display_builder_entity_view\EventSubscriber\DisplayBuilderSubscriber

display_builder_page_layout.display_builder_subscriber:
   class: Drupal\display_builder_page_layout\EventSubscriber\DisplayBuilderSubscriber

 display_builder_views.display_builder_subscriber:
   class: Drupal\display_builder_views\EventSubscriber\DisplayBuilderSubscriber

Other subscriptions:

display_builder_page_layout.page_variant_subscriber:
   class: Drupal\display_builder_page_layout\EventSubscriber\PageVariantSubscriber