Motivation for this change

This package allows Catalyst web projects to use relative URLs instead of absolute URLs. This is specifically interesting to me for Hydra, because using relative URLs allows using Hydra behind a proxy without extra configuration. I've tested Hydra with this package and it works well.

Once this package has been merged for a while, I'll send PRs to both the Hydra repo and to Nixpkgs to add this package as a dependency.

Pinging @volth for review of Perl, @grahamc and @edolstra for relevance to Hydra

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• [] Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.