Hot edit/customizepkg during dependency resolution

Customizing PKGBUILDs happens in `packageBuildable`.

Advantages:
* Handlers are run once for each PKGBUILD. Before, target packages
  could be edited both before and after dependency resolution, which
  is a little annoying.
* If dependencies of dependencies are changed during editing,
  dependency resolution will still work correctly. It's unlikely,
  though.
* It's simpler.

Disadvantages:
* The user is not shown a full list of dependencies before being asked
  to edit them. Then again, you can just run `aura -Ad` beforehand.