Issue #2031717 by Berdir, swentel, fago: Make entity module not required.

core/config/schema/core.entity.data_types.schema.yml

core/config/schema/core.entity.schema.yml

core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php

.../lib/Drupal/Core/Config/Entity/ConfigEntityBundleBase.php

core/lib/Drupal/Core/Entity/ContentEntityForm.php

core/lib/Drupal/Core/Entity/Entity/EntityFormDisplay.php

core/lib/Drupal/Core/Entity/Entity/EntityFormMode.php

core/lib/Drupal/Core/Entity/Entity/EntityViewDisplay.php

core/lib/Drupal/Core/Entity/Entity/EntityViewMode.php

core/lib/Drupal/Core/Entity/EntityDisplayBase.php

core/lib/Drupal/Core/Entity/EntityDisplayModeBase.php

core/lib/Drupal/Core/Entity/EntityDisplayModeInterface.php

core/lib/Drupal/Core/Entity/EntityFormModeInterface.php

core/lib/Drupal/Core/Entity/EntityManager.php

core/lib/Drupal/Core/Entity/EntityViewBuilder.php

core/lib/Drupal/Core/Entity/EntityViewModeInterface.php

core/lib/Drupal/Core/Extension/ModuleHandler.php

...install/core.entity_view_mode.aggregator_item.summary.yml

...nfig/install/core.entity_view_mode.block_content.full.yml

...ules/block_content/src/Tests/BlockContentCreationTest.php