Revise dependency framework to take into account variations between distributions of CentOS, Debian, etc

[raphaelcohn]

Improvements

• Short term
  • Add a runtime option to disable the dependency framework (perhaps an environment variable or file containing a value - eg use the conf framework)
  • Split up paths.d, it will become unmanageable
• Medium Term
  • Allow different packages to be specified by version of a distribution (eg lzma for Debian 6 or CentOS 5 rather than xz-utils)
    • Integrate this into swaddle, too, perhaps by getting fatten to output a list of dependencies
    • Get swaddle to build CentOS 5, etc specific repositories & packages
      • Solves a problem with BugURL and RPM hash algo choices for CentOS 5
      • Allows us to use xz compression of packages for more modern RPM and Deb based systems
  • Allow different repository names per distro (eg pandoc for CentOS 5 is not in EPEL)
  • Allow extensions to the repository discovery logic, so we don't have to keep modifying core (important for RedHat-derived distros, which have a myriad ways of installing repos)
  • Version max / min / equal pinning
  • A local path folder with symlinks which allows installation overrides (eg /usr/lib/PROGRAM/path)
  • How to manage dependencies 'in development', eg swaddle dependening on a new discotheque for disk management?
    • Long Term
  • A much more useful alternatives framework, that can check for programs and fallback