packaging: use structuredAttrs

staging-next banned !structuredAttrs && separateDebugInfo && disallowedRequisites
due to weird output interactions. Enable structuredAttrs so we can build again.

Also, fix type confusion that makes stdenv explode (https://github.com/NixOS/nixpkgs/issues/422989).

Co-authored-by: eldritch horrors <pennae@lix.systems>
Change-Id: Ic0c773394ee79e10d427f27750d59892d6d1f1d1

package.nix

@@ -135,10 +135,10 @@ let
 
   # This could be the dtrace for macOS, etc, but I have no idea if it is
   # packaged or if it works.
-  dtrace-generator = lib.optional withDtrace systemtap-lix;
+  dtrace-generator = if withDtrace then systemtap-lix else null;
 
   # This is for sys/sdt.h
-  dtrace-headers = lib.optional withDtrace libsystemtap;
+  dtrace-headers = if withDtrace then libsystemtap else null;
 
   aws-sdk-cpp-nix =
     if aws-sdk-cpp == null then
@@ -201,6 +201,8 @@ assert (lintInsteadOfBuild -> lix-clang-tidy != null);
 stdenv.mkDerivation (finalAttrs: {
   inherit pname version;
 
+  __structuredAttrs = true;
+
   # python3.withPackages does not splice properly, see https://github.com/NixOS/nixpkgs/issues/305858
   lixPythonForBuild = python3.pythonOnBuildForHost.withPackages (
     p: