Allow other modules to alter existing plugin definitions

Good idea! Since these are already plugins though, we just need to add a call set our desired alter hook in our plugin manager class (\Drupal\field_permissions\Plugin\FieldPermissionType\Manager):

So in the __construct() method something like:

  $this->alterInfo('field_permissions_types')

This would make the alter hook hook_field_permissions_types_alter().

Disregard my above comment, I completely missed the second part of the patch where you had indeed done as I suggested already :)

Committed. Thanks!