build-dep command

Hi all, This patch adds a build-dep command to cabal-install, so cabal-install build-dep mypackage installs all the dependencies of mypackage, but not mypackage itself. There are at least a couple of problems with this patch: * I don't know what happens if you say cabal-install build-dep foo bar and bar is one of the transitive dependencies of foo. * Removing the packages from the install plan should probably do so directly, rather than converting the plan to a list and back again. But I'm sending it now so it doesn't just sit on my HD for eternity. Thanks Ian