Hot edit/customizepkg during dependency resolution

The first commit is prep work I was doing before. It doesn't change any behavior (sorry, behaviour), it will hopefully get merged down the line later, and rebasing to remove it was too hard.

For the second commit:
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 that someone would do this,
  though.
* It's simpler.

Disadvantages:
* The user is not shown a full list of dependencies before being asked
  to edit the PKGBUILDs of dependencies. Then again, you can just run `aura -Ad` beforehand.

What do you think?