Allow hooks to specify a 'file' to include for implementation code.

It'd be nice if an implementation has a lot of code in it's configuring and processing to be able to specify a separate file to include like:

/**
 * Implement hook_form_controller_info().
 */
function vertical_tabs_form_info() {
  $alterations['vertical_tabs_form'] = array(
    'title' => t('Vertical tabs'),
    'description' => t('Enables fieldsets to be collapsed into a vertical tab display.'),
    'form callback' => 'vertical_tabs_form_configure',
    'file' => drupal_get_path('module', 'vertical_tabs') . '/vertical_tabs.form.inc',
  );
  return $alterations;
}