Show that the channels key exists.

I had to go fishing to find out how to add a channel to PEAR (I wanted to install Drush that way instead of via a dummy Drupal installation).

...although actually there seems to be some problems with it. Maybe that's why you didn't include it? It seems to try discovering channels twice on a re-provision. Let me dig in for a couple minutes and see if I can spot the issue...

That'll be a separate PR in any case.