NixOS-Config/modules/system/services/pc/keyd/default.nix

{ ... }:
{
  services.keyd = {
    enable = true;
    keyboards.default = {
      ids = [ "*" ];
      settings.main = {
        capslock = "overload(control, esc)";
        esc = "capslock";
      };
    };
  };
}