patch applied (cabal-install): Add the notion of paired packages to the Constraints ADT

[Duncan Coutts]

Sat Oct  4 18:31:41 PDT 2008  Duncan Coutts <duncan at haskell.org>
  * Add the notion of paired packages to the Constraints ADT
  Packages like base-3 and base-4 are paired. This means they are
  supposed to be treated equivalently in some contexts. Paired
  packages are installed packages with the same name where one
  version depends on the other.

    M ./Distribution/Client/Dependency/TopDown/Constraints.hs -11 +39

View patch online:
http://darcs.haskell.org/cabal-install/_darcs/patches/20081005013141-adfee-f4318b977b45b6b3d350e271957bbbca4d96df36.gz