Include XDG_DATA_DIRS/share/zsh/site-functions in FPATH by default?

[Jade Lovelace <jadel@xxxxxxxxxxx>]

It's conventional in some distro packaging such as on NixOS to put the
completions in a directory in $XDG_DATA_DIRS/share/zsh/site-functions.
Currently NixOS injects a zshenv which contains:

if [[ -z "${__ETC_ZSHENV_SOURCED:-}" ]]; then
    # Tell zsh how to find installed completions.
    for p in ''${(z)NIX_PROFILES}; do
        fpath=($p/share/zsh/site-functions
$p/share/zsh/$ZSH_VERSION/functions $p/share/zsh/vendor-completions
$fpath)
    done
fi

Nix will put each package's share directory in XDG_DATA_DIRS when
creating ephemeral shell environments as well.

This is superfluous in bash which uses XDG_DATA_DIRS by default:
https://github.com/scop/bash-completion/blob/51b009f6a653c66454816e9bc869c40f61329764/README.md?plain=1#L362-L372
and https://github.com/scop/bash-completion/blob/51b009f6a653c66454816e9bc869c40f61329764/bash_completion#L3306-L3309.

I am wondering if zsh has considered doing this by default as well, as
it makes zsh integrate cleanly with other tooling that uses
XDG_DATA_DIRS without specially needing to consider adding zsh
specific code to handle FPATH.

Regards,
Jade