WIP JSON impls without store dir

If we depend on the store dir, our JSON serializers/deserializers take
extra arguements, and that interfaces with the likes of various
frameworks for associating these with types (e.g. nlohmann in C++, Serde
in Rust, and Aeson in Haskell).

Author: Ericson2314

src/libstore/include/nix/store/derived-path.hh

@@ -304,4 +304,11 @@ typedef std::vector<DerivedPath> DerivedPaths;
  */
 void drvRequireExperiment(
     const SingleDerivedPath & drv, const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
+
 } // namespace nix
+
+JSON_IMPL(nix::DerivedPathOpaque)
+JSON_IMPL(nix::SingleDerivedPathBuilt)
+JSON_IMPL(nix::SingleDerivedPath)
+JSON_IMPL(nix::DerivedPathBuilt)
+JSON_IMPL(nix::DerivedPath)