JSON for Hash now has to be Base16

[Ericson2314]

Motivation

Fix #14532.

Context

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.)

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[roberth]

Did we accidentally duplicate the derivation outputs fixed output output format earlier?

...
    "name": "bash-5.3.tar.gz",
    "outputs": {
      "out": {
        "hash": {
          // howdy
          "algorithm": "sha256",
          "format": "base64",
          "hash": "DVzYaWX4aaJs9k9Lcb57lvkKO6iz104n6OnZ1VUPMbo="
        },
        "method": "flat"
      }
    },

[Ericson2314]

That was intentional, though I am open to better names.

it is supposed to be a pair of a method and a hash, and hashes contains a field called hash now.