Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/abstract-setting-to-json.hh

@@ -0,0 +1,18 @@
+#pragma once
+///@file
+
+#include <nlohmann/json.hpp>
+#include "config.hh"
+#include "json-utils.hh"
+
+namespace nix {
+template<typename T>
+std::map<std::string, nlohmann::json> BaseSetting<T>::toJSONObject() const
+{
+    auto obj = AbstractSetting::toJSONObject();
+    obj.emplace("value", value);
+    obj.emplace("defaultValue", defaultValue);
+    obj.emplace("documentDefault", documentDefault);
+    return obj;
+}
+}