Issue #1785974 by tim.plunkett, alexpott, sun: Move ConfigEntity into a Core component.

core/includes/module.inc

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

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

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

core/modules/config/tests/config_test/config_test.info

core/modules/config/tests/config_test/config_test.module

...g/tests/config_test/lib/Drupal/config_test/ConfigTest.php