As discussed today at great length in the Nix meeting, we don't want to
break the format, but we also don't want to impede the improvement of
JSON formats. The solution is to add a new flag for control the output
format.
Note that prior to the release, we may want to replace `--json
--json-format N` with `--json=N`, but this is being left for a separate
PR, as we don't yet have `=` support for CLI flags.
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.
