How to find where Drush is installed to include a file in my module?
A temporary solution that I found is define the same constants in the drush_help.module file.

/**
 * Constants defined by drush in the includes/bootstrap.inc file.
 *
 * To see why we need to do this please read:
 * https://drupal.stackexchange.com/q/250786/28275
 */
define('DRUSH_BOOTSTRAP_NONE', -1);
define('DRUSH_BOOTSTRAP_MAX', -2);
define('DRUSH_BOOTSTRAP_DRUSH', 0);
define('DRUSH_BOOTSTRAP_DRUPAL_ROOT', 1);
define('DRUSH_BOOTSTRAP_DRUPAL_SITE', 2);
define('DRUSH_BOOTSTRAP_DRUPAL_CONFIGURATION', 3);
define('DRUSH_BOOTSTRAP_DRUPAL_DATABASE', 4);
define('DRUSH_BOOTSTRAP_DRUPAL_FULL', 5);
define('DRUSH_BOOTSTRAP_DRUPAL_LOGIN', 6);