The old cloud-only mappings cannot express root-level cache and client
settings. Expose complete YAML documents while preserving existing
configurations through standard rename aliases.
Nixpkgs switched pkgs.formats.yaml to remarshal v2. Generated files now
start with a %YAML 1.1 directive and a --- document start, and multiline
strings use double-quoted escapes.
Only files written through yamlFormat.generate change. Modules that
declare the YAML type but write the file with builtins.toJSON keep their
current output.
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
