These files feature simple aliases and a nix-shell detector when using
`nix-shell --run zsh`. The package itself contains the completion scripts in
`$out/share/zsh/site-functions` (to keep it compatible with ZSH-only
setups) and the plugins in `$out/share/zsh/plugins` for oh-my-zsh.

This ensures that no impurity exists due to implicitly depending
on`/usr/bin/env`.

It stores the theme into `$out/share/zsh/themes` to make it possible to
find theme using `buildEnv` and remain consistent with other ZSH
extensions.

…s for `ZSH_CUSTOM`

If multiple third-party modules shall be used for `oh-my-zsh` it has to
be possible to create another env which composes all the packages.

Now it can be done like this:

```
{ pkgs, ... }:
{
  programs.zsh.enable = true;
  programs.zsh.ohMyZsh = {
    enable = true;
    customPkgs = with pkgs; [
      lambda-mod-zsh-theme
      nix-zsh-completions
    ];
    theme = "lambda-mod";
    plugins = [ "nix" ];
  };
}
```

Please keep in mind that this is not compatible with
`programs.zsh.ohMyZsh.custom`, only one of these options can be used
ATM.

Each package should store its outputs into
`$out/share/zsh/<output-name>`. Completions (and ZSH-only) extensions
should live in the `fpath` (`$out/share/zsh/site-functions`), plugins in
`.../plugins` and themes in `.../themes` (please refer to
fdb6bf6ed68c2f089ae6c729dfeaa3eddea2ce6a and 406d64aad162b3a4881747be4e24705fb5182573).

All scripts in `customPkgs` will be linked together using `linkFarm` to
provide a single directory for all scripts from all derivations in
`customPkgs` as suggested in #43282 (comment).

In the last year `programs.oh-my-zsh` gained more complexity and since
the introduction of features like `customPkgs` which builds a
`ZSH_CUSTOM` path from a sequence of derivation a documentation may be
fairly helpful to make the knowledge how to use the module and how to
package new ZSH plugins visible.

See #43282 (comment)

nixos/ohMyZsh: allow multiple derivations in `ZSH_CUSTOM`