Issue #2137301 by cayci1, fago: Move entity constraints under the Entity component.

.../Entity/Plugin/Validation/Constraint/BundleConstraint.php

...lugin/Validation/Constraint/BundleConstraintValidator.php

.../Plugin/Validation/Constraint/EntityChangedConstraint.php

...alidation/Constraint/EntityChangedConstraintValidator.php

...ity/Plugin/Validation/Constraint/EntityTypeConstraint.php

...n/Validation/Constraint/EntityTypeConstraintValidator.php

...Plugin/Validation/Constraint/ValidReferenceConstraint.php

...lidation/Constraint/ValidReferenceConstraintValidator.php