JSON for Hash now has to be Base16

Fix #14532.

As discussed on the call today:

1. We'll stick with `format = "base16"` and `hash = "<hash>"`, not do
   `base16 = "<hash>"`, in order to be forward compatible with
   supporting more versioning formats.

   The motivation we discussed for someday *possibly* doing this is
   making it easier to write very slap-dash lang2nix tools that create
   (not consume) derivations with dynamic derivations.

2. We will remove support for non-base16 (and make that the default, not
   base64) in `Hash`, so this is strictly forward contingency, *not*
   yet something we support. (And also not something we have concrete
   plans to start supporting.)

tests/functional/path-info.sh

@@ -19,13 +19,13 @@ diff --unified --color=always \
         {
           "$foo": {
             "algorithm": "sha256",
-            "format": "base64",
-            "hash": "QvtAMbUl/uvi+LCObmqOhvNOapHdA2raiI4xG5zI5pA="
+            "format": "base16",
+            "hash": "42fb4031b525feebe2f8b08e6e6a8e86f34e6a91dd036ada888e311b9cc8e690"
           },
           "$bar": {
             "algorithm": "sha256",
-            "format": "base64",
-            "hash": "9fhYGu9fqxcQC2Kc81qh2RMo1QcLBUBo8U+pPn+jthQ="
+            "format": "base16",
+            "hash": "f5f8581aef5fab17100b629cf35aa1d91328d5070b054068f14fa93e7fa3b614"
           },
           "$baz": null
         }