Refactoring: change the return type of 'InstallPlan.ready'.

Introduce a new type 'ReadyPackage' to represent packages that have all
dependencies already installed. Make 'InstallPlan.ready' return '[ReadyPackage]'
instead of '[(ConfiguredPackage, InstalledPackageInfo)]'.

Also fix a bug where 'cabal configure' didn't pass '--dependency' options to
'setup configure'.