nixos/oh-my-zsh: add customPkgs option to allow multiple derivations 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 https://github.com/NixOS/nixpkgs/pull/43282#issuecomment-410396365.

nixos/modules/programs/zsh/oh-my-zsh.nix

@@ -3,7 +3,30 @@
 with lib;
 
 let
+
   cfg = config.programs.zsh.ohMyZsh;
+
+  mkLinkFarmEntry = name: dir:
+    let
+      env = pkgs.buildEnv {
+        name = "zsh-${name}-env";
+        paths = cfg.customPkgs;
+        pathsToLink = "/share/zsh/${dir}";
+      };
+    in
+      { inherit name; path = "${env}/share/zsh/${dir}"; };
+
+  mkLinkFarmEntry' = name: mkLinkFarmEntry name name;
+
+  custom =
+    if cfg.custom != null then cfg.custom
+    else if length cfg.customPkgs == 0 then null
+    else pkgs.linkFarm "oh-my-zsh-custom" [
+      (mkLinkFarmEntry' "themes")
+      (mkLinkFarmEntry "completions" "site-functions")
+      (mkLinkFarmEntry' "plugins")
+    ];
+
 in
   {
     options = {
@@ -34,10 +57,19 @@ in
         };
 
         custom = mkOption {
-          default = "";
-          type = types.str;
+          default = null;
+          type = with types; nullOr str;
           description = ''
             Path to a custom oh-my-zsh package to override config of oh-my-zsh.
+            (Can't be used along with `customPkgs`).
+          '';
+        };
+
+        customPkgs = mkOption {
+          default = [];
+          type = types.listOf types.package;
+          description = ''
+            List of custom packages that should be loaded into `oh-my-zsh`.
           '';
         };
 
@@ -67,7 +99,7 @@ in
 
       environment.systemPackages = [ cfg.package ];
 
-      programs.zsh.interactiveShellInit = with builtins; ''
+      programs.zsh.interactiveShellInit = ''
         # oh-my-zsh configuration generated by NixOS
         export ZSH=${cfg.package}/share/oh-my-zsh
 
@@ -75,8 +107,8 @@ in
           "plugins=(${concatStringsSep " " cfg.plugins})"
         }
 
-        ${optionalString (stringLength(cfg.custom) > 0)
-          "ZSH_CUSTOM=\"${cfg.custom}\""
+        ${optionalString (custom != null)
+          "ZSH_CUSTOM=\"${custom}\""
         }
 
         ${optionalString (stringLength(cfg.theme) > 0)
@@ -92,5 +124,13 @@ in
 
         source $ZSH/oh-my-zsh.sh
       '';
+
+      assertions = [
+        {
+          assertion = cfg.custom != null -> cfg.customPkgs == [];
+          message = "If `cfg.custom` is set for `ZSH_CUSTOM`, `customPkgs` can't be used!";
+        }
+      ];
+
     };
   }