nix-zsh-completions package and module support

[spwhitt]

1. Adds a package for nix-zsh-completions
2. Configures zsh's fpath to include completions installed through Nix packages
3. Adds an enableCompletion option ZSH module

I set up the enableCompletion option to also pull in the nix-zsh-completions package, under the assumption that anyone using completions on NixOS is probably interested in completions for the Nix utilities as well. If anyone doesn't like this, feel free to let me know and I'll remove that line, and people will have to add nix-zsh-completions to their systemPackages manually.

@jagajaga @offlinehacker @garbas

[mention-bot]

By analyzing the blame information on this pull request, we identified @oxij, @edolstra and @ttuegel to be potential reviewers.

[jagajaga]

In my opinion True as a default here is better :)

[spwhitt]

If you say so :)

[jagajaga]

👍

[globin]

see #11155 (enable shared site-function in pathsToLink)

[spwhitt]

@abbradar thanks - it must have worked for me because my desktop environment links /share. The actual required path is /share/zsh/$ZSH_VERSION/functions so I'm guessing I should just link all of /share/zsh since I won't be able to use $ZSH_VERSION inside pathsToLink - is this correct?

[abbradar]

Strange, it works for me with /share/zsh/functions but I guess there's both that and $ZSH_VERSION path so we actually need four paths to add to pathsToLink. For $ZSH_VERSION -- what do you think of using lib.getVersion zsh to make this path static?

[spwhitt]

My system doesn't have /share/zsh/functions and if yours does it's either a bug in some other package or it needs to be added to $fpath as well... using lib.getVersion sounds brittle, simple formatting changes could break it. /share/zsh doesn't have anything we don't want to link anyway (in the zsh package), so linking the whole thing shouldn't effect performance. I argume we play it safe and link /share/zsh

[abbradar]

nix-zsh-completions uses this path, for example. Also my fpath already contains needed paths, added by the very spwhitt@ff58711#diff-0740f57e63af61694d14796286cb9204R115 ^_^.

OK, let's link the whole thing -- shouldn't hurt.

[abbradar]

Oh, sorry, it uses /share/zsh/site-functions -- sleepy me.