Make nix path-info --json return an object not array

Before it returned a list of JSON objects with store object information,
including the path in each object. Now, it maps the paths to JSON
objects with the metadata sans path.

This matches how `nix derivation show` works.

Quite hillariously, none of our existing functional tests caught this
change to `path-info --json` though they did use it. So just new
functional tests need to be added.

src/libstore/parsed-derivations.cc

@@ -148,6 +148,11 @@ static nlohmann::json pathInfoToJSON(
         auto & jsonPath = jsonList.emplace_back(
             info->toJSON(store, false, HashFormat::Base32));
 
+        // Add the path to the object whose metadata we are including.
+        jsonPath["path"] = store.printStorePath(storePath);
+
+        jsonPath["valid"] = true;
+
         jsonPath["closureSize"] = ({
             uint64_t totalNarSize = 0;
             StorePathSet closure;