Problem/Motivation
Number, email, and reference field types are defined in the Core library, and when it came time to create MigrateField plugins for these fields, they were placed in the Core library.
However, these plugins extend MigrateFieldPluginBase from the migrate_drupal module, and depend on Interfaces from migrate and migrate_drupal modules. Code that depends on code in core modules should not be located in the Core Libraries.
Proposed resolution
Move plugins supporting field migrations of core fields into the field module, configure deprecations on the current classes in case anyone has extended them.
Remaining tasks
Do it.
User interface changes
none
API changes
None, but location of the plugins will change. This is allowed per BC policy, but we can leave deprecated BC shims in place until Drupal 9.
Data model changes
None.
