Check required directory structure on module install/enable
The Drupal API function drupal_load() will let you preload your .module file so you can make its constants/functions available during your hook_requirements() invocation:

/**
 * Implements hook_requirements().
 */
function MYMODULE_requirements($phase) {

  // @TODO: misc code.

  drupal_load('module', 'MYMODULE');

  // @TODO: code that requires MYMODULE_CONST_DEFINITION
}