Shuffle ValidPathInfo JSON rendering

`Store::pathInfoToJSON` was a rather baroque functions, being full of
parameters to support both parsed derivations and `nix path-info`. The
common core of each, a simple `dValidPathInfo::toJSON` function, is
factored out, but the rest of the logic is just duplicated and then
specialized to its use-case (at which point it is no longer that
duplicated).

This keeps the human oriented CLI logic (which is currently unstable)
and the core domain logic (export reference graphs with structured
attrs, which is stable), separate, which I think is better.

src/libstore/path-info.hh

@@ -125,6 +125,18 @@ struct ValidPathInfo : UnkeyedValidPathInfo {
 
     Strings shortRefs() const;
 
+    /**
+     * @param includeImpureInfo If true, variable elements such as the
+     * registration time are included.
+     */
+    virtual nlohmann::json toJSON(
+        const Store & store,
+        bool includeImpureInfo,
+        HashFormat hashFormat) const;
+    static ValidPathInfo fromJSON(
+        const Store & store,
+        const nlohmann::json & json);
+
     ValidPathInfo(const ValidPathInfo & other) = default;
 
     ValidPathInfo(StorePath && path, UnkeyedValidPathInfo info) : UnkeyedValidPathInfo(info), path(std::move(path)) { };