Handle cases where repo and owner are not literal strings

[expipiplus1]

like

stdenv.mkDerivation rec {
  pname = "monado";
  version = "21.0.0";

  src = fetchFromGitLab {
    domain = "gitlab.freedesktop.org";
    owner = pname;
    repo = pname;
    rev = "v${version}";
    sha256 = "07zxs96i3prjqww1f68496cl2xxqaidx32lpfyy0pn5am4c297zc";
  };