Restore subdir support in registries

Hacky...

src/libexpr/common-eval-args.cc

@@ -42,9 +42,11 @@ MixEvalArgs::MixEvalArgs()
       .description("override a flake registry value")
       .arity(2)
       .handler([&](std::vector<std::string> ss) {
-          fetchers::overrideRegistry(
-              parseFlakeRef(ss[0], absPath(".")).input,
-              parseFlakeRef(ss[1], absPath(".")).input);
+          auto from = parseFlakeRef(ss[0], absPath("."));
+          auto to = parseFlakeRef(ss[1], absPath("."));
+          fetchers::Input::Attrs extraAttrs;
+          if (to.subdir != "") extraAttrs["subdir"] = to.subdir;
+          fetchers::overrideRegistry(from.input, to.input, extraAttrs);
       });
 }
 

src/libexpr/flake/flakeref.cc

@@ -40,7 +40,8 @@ bool FlakeRef::operator ==(const FlakeRef & other) const
 
 FlakeRef FlakeRef::resolve(ref<Store> store) const
 {
-    return FlakeRef(lookupInRegistries(store, input), subdir);
+    auto [input2, extraAttrs] = lookupInRegistries(store, input);
+    return FlakeRef(input2, fetchers::maybeGetStrAttr(extraAttrs, "subdir").value_or(subdir));
 }
 
 FlakeRef parseFlakeRef(

src/libexpr/primops/fetchTree.cc

@@ -36,7 +36,7 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
         input = fetchers::inputFromURL(state.coerceToString(pos, *args[0], context, false, false));
 
     if (!evalSettings.pureEval && !input->isDirect())
-        input = lookupInRegistries(state.store, input);
+        input = lookupInRegistries(state.store, input).first;
 
     if (evalSettings.pureEval && !input->isImmutable())
         throw Error("in pure evaluation mode, 'fetchTree' requires an immutable input");