we need the file itself as a dependency for the docbook build, but we don't need
it to be properly sorted at the nix level. push the sort out to a python script
instead to save eval time. on the machine used to write this `nix-instantiate
<nixos/nixos> -A system` went down from 7.1s to 5.4s and GC heap size decreased
by 50MB (or 70MB max RSS).
When using `documentation.nixos.includeAllModules = true;` with external
modules, the string context might contain dependencies to derivations
and so `toFile` refuses to evaluate;
```
error: in 'toFile': the file 'options.xml' cannot refer to derivation outputs, at
[...]/nixpkgs/nixos/lib/make-options-doc/default.nix:89:16
```
This is not an issue when using `writeText` (instead of manually
stripping the context).
Motivation is to support other repositories containing nixos
modules that would like to generate options documentation:
- nix-darwin
- private repos
- arion
- ??
