programs.zsh.ohMyZsh: add `package` option to make package overrides on module-base easier

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

···
           '';
         };
 
+        package = mkOption {
+          default = pkgs.oh-my-zsh;
+          defaultText = "pkgs.oh-my-zsh";
+          description = ''
+            Package to install for `oh-my-zsh` usage.
+          '';
+
+          type = types.package;
+        };
+
         plugins = mkOption {
           default = [];
           type = types.listOf(types.str);
···
       # Prevent zsh from overwriting oh-my-zsh's prompt
       programs.zsh.promptInit = mkDefault "";
 
-      environment.systemPackages = with pkgs; [ oh-my-zsh ];
+      environment.systemPackages = [ cfg.package ];
 
-      programs.zsh.interactiveShellInit = with pkgs; with builtins; ''
+      programs.zsh.interactiveShellInit = with builtins; ''
         # oh-my-zsh configuration generated by NixOS
-        export ZSH=${oh-my-zsh}/share/oh-my-zsh
+        export ZSH=${cfg.package}/share/oh-my-zsh
 
         ${optionalString (length(cfg.plugins) > 0)
           "plugins=(${concatStringsSep " " cfg.plugins})"