Rework "Generate unbound configuration files from blocklist files (#2833)"

Hi, I took the changes from #2833 and reworked them a bit. Notably this: - moves package definitions to a separate `packages.nix` file, making this repository useable by non flake users (as recommended by @toastal [here](https://github.com/StevenBlack/hosts/pull/2833#issuecomment-3832965693) (although I settled for `packages.nix` instead of `overlays.nix`)) - splits package defintions into `raw` and `unbound` sets, raw containing the original `hosts` files (this allows nix users to create derivations of the original files, adjusting them for other applications as required) - uses a shorter `local-zone: "${domain}" refuse` unbound config notation (where applicable), shrinking unbound config size by ~50% This depends on #3076 (it currently includes the changes from #3076 and I'll rebase once merged).