Handle content type migrations where not all product-display nodes are products

One thing you'll find is that if content type migration for product nodes are removed, to migrate them you need to disable one of the hooks:

/**
 * Implements hook_module_implements_alter()
 */
function MYMODULE_module_implements_alter(&$implementations, $hook) {
  // Disable commerce_migrate_commerce_migration_plugins_alter() so that the
  // product content type migration can be used.
  if ($hook == 'hook_migration_plugins_alter') {
    unset($implementations['commerce_migrate_commerce']);
  }
}