Make the JSON format for derivation use basename store paths

See #13570 for details --- the idea is that included the store dir in
store paths makes systematic JSON parting with e.g. Serde, Aeson,
nlohmann, or similiar harder.

After talking to Eelco, we are changing the `Derivation` format right
away because not only is `nix derivation` technically experimental, we think it is
also less widely used in practice than, say, `nix path-info`.

Progress on #13570

src/libstore/path.cc

@@ -1,4 +1,7 @@
+#include <nlohmann/json.hpp>
+
 #include "nix/store/store-dir-config.hh"
+#include "nix/util/json-utils.hh"
 
 namespace nix {
 
@@ -75,3 +78,19 @@ StorePath StorePath::random(std::string_view name)
 }
 
 } // namespace nix
+
+namespace nlohmann {
+
+using namespace nix;
+
+StorePath adl_serializer<StorePath>::from_json(const json & json)
+{
+    return StorePath{getString(json)};
+}
+
+void adl_serializer<StorePath>::to_json(json & json, StorePath storePath)
+{
+    json = storePath.to_string();
+}
+
+} // namespace nlohmann