Introduce

    dependencyClosure :: InstallPlan
                      -> [PackageIdentifier]
                      -> Either (PackageIndex PlanPackage) [(PlanPackage, [InstalledPackageId])]

And use this in the definition of `pruneInstallPlan` in `freeze`, to avoid
first converting an install plan from a `Cabal.PackageIndex` to a
`CabalInstall.PackageIndex`.

This resolves the first of the two irregularities mentioned in the previous
commit.