v0.16.8 - 'keyed apt source'; Array#val_for; more Dep refactoring.

* Added 'keyed apt source' dep, to add custom apt sources along with their keys.
* The logic in String#val_for actually works with an array, so it's been moved
  to Array#val_for (which makes it available on both).
* Lots of decoupling refactors around Dep/Task/DepCache.