perform package dependency analysis for cabal-install

[ijones]

This will require some refactoring, since we'll have to unpack all of the packages on the command-line first, then read their .cabal files to get the dependencies.

Also look at what's installed and whether or not all dependencies are met. cabal-get implements this already; should be refactored to move this code here, and cabal-get should call cabal-install.

[nominolo]

Obsolete.