support for resolving dependencies using systems other than apt

It would be great to support non-apt ways of installing system packages. We should probably track each of these systems (AUR, RPM, etc) in their own issue.

The requirement for supporting a packaging system is that there is some way of converting abstract requirements to packages to install. For example, a LibraryRequirement('libsdl', minimum_version='1.2') needs to be converted to a RPM that satisfies that dependency.

My familiarity is primarily with APT; I'd love to work with experts from other distributions on supporting other packaging systems.