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.

tests/functional/path-info.sh

@@ -0,0 +1,23 @@
+source common.sh
+
+echo foo > $TEST_ROOT/foo
+foo=$(nix store add-file $TEST_ROOT/foo)
+
+echo bar > $TEST_ROOT/bar
+bar=$(nix store add-file $TEST_ROOT/bar)
+
+echo baz > $TEST_ROOT/baz
+baz=$(nix store add-file $TEST_ROOT/baz)
+nix-store --delete "$baz"
+
+diff --unified --color=always \
+    <(nix path-info --json "$foo" "$bar" "$baz" |
+        jq --sort-keys 'map_values(.narHash)') \
+    <(jq --sort-keys <<-EOF
+        {
+          "$foo": "sha256-QvtAMbUl/uvi+LCObmqOhvNOapHdA2raiI4xG5zI5pA=",
+          "$bar": "sha256-9fhYGu9fqxcQC2Kc81qh2RMo1QcLBUBo8U+pPn+jthQ=",
+          "$baz": null
+        }
+EOF
+    )