Issue #1875996 by Xano, tim.plunkett, neclimdul, Rajendar Reddy, Jalandhar: Reconsider naming conventions for derivative classes.

Issue #2165155 by Berdir, tim.plunkett: Change $entity_type to $entity_type_id and $entity_info to $entity_type/$entity_types.

Issue #2005716 by tim.plunkett, msonnabaum, dawehner, alexpott, effulgentsia: Promote EntityType to a domain object.

Issue #2026769 by fgm: Fixed Simplify DerivativeInterface implementations by refactoring duplicated code to a base class.

Issue #1950632 by effulgentsia, Wim Leers, amateescu, yched: Create a FieldDefinitionInterface, decoupled from  and  config entities, and use it for formatters and widgets.

Issue #1801304 by Amitaibu, amateescu, effulgentsia, tim.plunkett, EclipseGc, Fabianx, Cottser: Added Entity reference field.