Introduce a new superclass HasInstalledPackageId:

    class Package pkg => HasInstalledPackageId pkg where
      installedPackageId :: pkg -> InstalledPackageId

    class HasInstalledPackageId pkg => PackageInstalled pkg where
      installedDepends :: pkg -> [InstalledPackageId]

Most functions that deal with the package index now just require
HasInstalledPackageId; only the functions that actually require the
dependencies still rely on PackageInstalled.

The point is that a ConfiguredPackage/ReadyPackage/PlanPackage can reasonably
be made an instance of HasInstalledPackageId, but not of PackageInstalled; that
will be the topic of the next, much larger, pull request.