Missing mappings for "nodereference_buttons" & "nodereference_autocomplete" widgets

Problem/Motivation

Migrating D6 to D8
When migrating node reference fields from a D6 site to a D8 site, I get the following error:

The "nodereference_buttons" plugin does not exist. (\core\lib\Drupal\Component\Plugin\Discovery\DiscoveryTrait.php:52)

In nodereference.mode, there's 3 widget types:

function nodereference_widget_info() {
  return array(
    'nodereference_select' => array([...]),
    'nodereference_buttons' => array([...]),
    'nodereference_autocomplete' => array([...]),
    ),
  );
}

But in d6_field_instance_widget_settings.yml, we only have a mapping for nodereference_select:

  'options/type':
    type:
      plugin: static_map
      bypass: true
      source: widget_type
      map:
        number: number
        email_textfield: email_default
        date_select: datetime_default
        date_text: datetime_default
        date_popup: datetime_default
        imagefield_widget: image_image
        phone_textfield: telephone_default
        optionwidgets_onoff: boolean_checkbox
        optionwidgets_buttons: options_buttons
        optionwidgets_select: options_select
        nodereference_select: options_select
        userreference_select: options_select

Proposed resolution

Add the missing mappings:

• nodereference_buttons
• nodereference_autocomplete

Remaining tasks

Write a patch
Review

User interface changes

None.

API changes

None.

Data model changes

None.

Problem