Use drush_log($msg, 'ok') instead of drush_print()

When you are emitting information which is important for the logs, use drush_log() instead of drush_print(). This is helpful because command can be executed remotely and drush_log() insures that your log info gets passed back to the caller correctly.

For example, the 'package downloaded' log info should be:

OLD: drush_print("{$this->project->name} downloaded from {$this->project->download['url']}.");
NEW: drush_log("{$this->project->name} downloaded from {$this->project->download['url']}.", 'ok');

Note that you can and should put random debug info into the logs using 'notice' log level. Thats a separate issue.