Make the indirect fetcher input scheme part of the Flakes XP feature

I don't know much about it, but by the number of times "flake" appears
in the code it seems like is part of flakes, at least for now.

src/libfetchers/indirect.cc

@@ -98,6 +98,11 @@ struct IndirectInputScheme : InputScheme
     {
         throw Error("indirect input '%s' cannot be fetched directly", input.to_string());
     }
+
+    std::optional<ExperimentalFeature> experimentalFeature() override
+    {
+        return Xp::Flakes;
+    }
 };
 
 static auto rIndirectInputScheme = OnStartup([] { registerInputScheme(std::make_unique<IndirectInputScheme>()); });