Use drupal_load() when it's possible

mongodb_cache.inc can be used as a library (loads in settings.php) and doesn't need to be inside Drupal path. But then if people still want to enable Mongodb module, it ends up with including a same mongodb.module file from different locations.

I propose changing the line

include_once dirname(__FILE__) . '/../mongodb.module';

to

if (!drupal_load('module', 'mongodb')) {
  include_once dirname(__FILE__) . '/../mongodb.module';
}

The other advantages would be that drupal_load() will find the right location and prevent duplicate include_once.

I'll upload a patch.

There are more include_once in other sub-modules, mongodb_session and mongodb_watchdog. I think it would be better to use drupal_load when it's possible.