patch applied (cabal-install): Implement plan improvement

[Duncan Coutts]

Sun Jun  1 17:00:18 PDT 2008  Duncan Coutts <duncan at haskell.org>
  * Implement plan improvement
  The idea is to improve the plan by swapping a configured package for
  an equivalent installed one. For a particular package the condition
  is that the package be in a configured state, that a the same version
  be already installed with the exact same dependencies and all the
  packages in the plan that it depends on are in the installed state.

    M ./Hackage/Dependency/TopDown.hs -9 +55

View patch online:
http://darcs.haskell.org/cabal-install/_darcs/patches/20080602000018-adfee-b91222fbe160ac21b1712f55dda8e5a5fa6de392.gz