Dispatch ImportEntityEvent during content import

Problem/Motivation

\Drupal\Core\DefaultContent\PreImportEvent allows skipping content but nothing else. Having an event fired after $this->toEntity before validation would be great. So ImportEntityEvent? so things can do stuff before validating

Example: XB stores target_id in its component tree, not UUID. Probably other things do this too. So target IDs need to be fixed in dumps if the DefaultContent API's UUID -> ID resolution cannot be fully used

Steps to reproduce

Proposed resolution

Dispatch ImportEntityEvent per-entity once populated and before validation

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet