most modules can be evaluated for their documentation in a very
restricted environment that doesn't include all of nixpkgs. this
evaluation can then be cached and reused for subsequent builds, merging
only documentation that has changed into the cached set. since nixos
ships with a large number of modules of which only a few are used in any
given config this can save evaluation a huge percentage of nixos
options available in any given config.
in tests of this caching, despite having to copy most of nixos/, saves
about 80% of the time needed to build the system manual, or about two
second on the machine used for testing. build time for a full system
config shrank from 9.4s to 7.4s, while turning documentation off
entirely shortened the build to 7.1s.
The upstream pipewire config is written in an almost, but not quite
JSON format. The parser accepts standard JSON, though, so we don't
need to write our file in the same nonstandard version.
The typing for all config options is changed from `types.attrs`, which
behaves poorly when the option is set from multiple locations, to the
formats.json-type.
Also, rewrite some very long one-liners for improved readability.
