Support for Field Formatters and Field Widgets defined for core Entity Reference fields

Problem/Motivation

There are many modules that provide Field Formatter and Field Widget plugins for core Entity Reference fields, but they don't support Primary Entity Reference fields because they explicitly specify the "entity_reference" field type ID.

Steps to reproduce

Proposed resolution

Implement alter hooks to insert ourselves.

This is the approach taken by the Group module, which we could easily adapt:

/**
 * Implements hook_field_formatter_info_alter().
 */
function primary_entity_reference_field_formatter_info_alter(array &$info) {
  // Anything that supports entity reference fields should also work for ours.
  foreach ($info as $key => $formatter_info) {
    if (in_array('entity_reference', $formatter_info['field_types'], TRUE)) {
      $info[$key]['field_types'][] = 'primary_entity_reference';
    }
  }
}

Remaining tasks

User interface changes

API changes

Data model changes