drush pm-enable cannot be hooked by commands that are being enabled

The devel hook drush_devel_post_enable (which should now be named drush_devel_post_pm_enable now) will never be called (even if correctly renamed), because drush now will only allow enabled modules to participate in command hooks. All modules, enabled or no, were formerly included, so this used to work.

There needs to be a good way for modules to automatically download their dependencies. We could punt on this and recommend using the Drupal dependency check for this, but putting it in a drush hook as devel did has the advantage that you can show progress, prompt the user, etc.

We could attempt to make the pm-enable command explicitly add modules being enabled to the drush commandfile list (if they have drush commandfiles). This would be pretty easy to do in drush_init_pm_enable. Alternately, we could add special code to pm-download to give modules a chance.

I think I favor the former. Thoughts?