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.)
A few changes had cropped up with `_NIX_TEST_ACCEPT=1`:
1. Blake hashing test JSON had a different indentation
2. Store URI had improper non-quoted spaces
(1) was is just fixed, as we trust nlohmann JSON to parse JSON
correctly, regardless of whitespace.
For (2), the existing URL was made a read-only test, since we very much
wish to continue parsing such invalid URLs directly. And then the
original read/write test was updated to properly percent-encode the
space, as the normal form should be.
