Problem/Motivation
Widgets and Formatters are now agnostic as to whether they are applied to field.module fields or base fields, so their plugin managers have no business in field.module anymore, and have been moved to a Core/Field system.
The FieldType plugin manager has been moved to the entity system, but its discovery still looks for classes in 'Plugin/field/field_type' folders.
API Changes
1) the FieldType plugin manager moves to the Core/Field system (from Core/Entity/Field) (Together with all other classes in the Drupal\Core\Entity\Field namespace, namely FieldItem, FieldItemList and their interfaces).
2) Service ID for the field_type plugin manager changes from $container->get('plugin.manager.entity.field.field_type') to $container->get('plugin.manager.field.field_type').
2) The FieldFormatter and FieldWidget plugin managers move from field.module to Core/Field as well. Their service ID's did not change.
3) Folders for discovery of widget, formatter, field type plugins change
For widget, it changes from 'Plugin/field/widget' to 'Plugin/Field/FieldWidget';
For formatter, it changes from 'Plugin/field/formatter' to 'Plugin/Field/FieldFormatter';
For field_type, it changes from 'Plugin/field/field_type' to 'Plugin/Field/FieldType';
4. Base classes and Interfaces for Widget and Formatter plugins were moved to \Drupal\Core\Field as well.
