Add index-state support to horizon-spec

Adds support for reproducible package resolution using index-state timestamps, similar to cabal.project's index-state feature.

Schema Changes

HaskellSource type:

• Replace FromHackage with FromIndex { index : Text, name : Text, version : Version }
• The index field contains repository base URL (e.g., "https://hackage.haskell.org")

New types:

• IndexState = List { index : Text, timestamp : Text }
• ExportSettings base type with packagesDir and indexState fields

Export settings:

• PackageSetExportSettings: Now extends ExportSettings (includes indexState field)
• OverlayExportSettings: Now extends ExportSettings (includes indexState field)

New Helper Functions

• callIndex: For custom package indices with any repository URL
• callCHaP: Shortcut for Cardano Haskell Packages (https://chap.intersectmbo.org)
• callHackage: Updated to use FromIndex with https://hackage.haskell.org

How It Works

• Repository base URLs are used as identifiers (e.g., "https://hackage.haskell.org")
• horizon-gen-nix will fetch indices by appending /01-index.tar.gz to the URL
• Optional indexState configuration allows pinning package availability to specific timestamps
• This enables pure, reproducible package resolution

Dependencies

This MR is based on !15 (merged) (gitlab-ci 0.8.0 update) and can be merged after !15 (merged) is merged.