Standardize lts/ghc-9.8.x to use nix/ + import-tree
Summary
- Standardize to use nix/ directory structure (instead of flakeModules/ or inline patterns)
- Replace horizon-flake-parts bundle with individual inputs
- Use flake-parts with import-tree pattern
- Create modular configuration files
- Replace inline/old CI with remote gitlab-ci v0.3.0
- Add/update gitlab-ci-omnix input at v0.4.0
- Use config.horizon.systems.ci.host/targets for systems