Fixes#672 by using the first index-state that is > than desired
index state. It also passes `--index-state` to `cabal v2-cconfigre`
to make sure it cabal uses only the packages in the hackage
file that are before the specified date.
Fixes#540 by returning the `index-state` used back out of
`cabalProject` in the same way `plan-nix` is returned.