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.)
Since we haven't released v2 yet (2.32 has v1) we can just update this
in-place and avoid version churn.
Note that as a nice side effect of using the standard `Hash` JSON impl,
we don't neeed this `hashFormat` parameter anymore.
It turns out this code path is only used for unit tests (to ensure our
JSON formats are possible to parse by other code, elsewhere). No
user-facing functionality consumes this format.
Therefore, let's drop the old version parsing support.