Portage/Dependency.hs: simplify datastructure holding dependency ranges

- Removed most of horrible simplification code I wrote long ago :]
- Tuned prettyprinter to print-out better nested USE flags.
  Now generates less newlines
- Added basic prettyprinter test
  Now it thoroughly tests cases for empty depends

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>