Move /src to /subprojects

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

subprojects/libexpr/json-to-value.hh

@@ -0,0 +1,17 @@
+#pragma once
+///@file
+
+#include "error.hh"
+
+#include <string>
+
+namespace nix {
+
+class EvalState;
+struct Value;
+
+MakeError(JSONParseError, Error);
+
+void parseJSON(EvalState & state, const std::string_view & s, Value & v);
+
+}