Issue description

If I have an option "foo" of type attrsOf (submodule bar) and I assign an attrset with _module.args.name "bar", the configuration of submodule bar will see its name as "foobar".

This is because _module.args is types.unspecified which follows the default merging rules, but I'm not sure how to fix it (or if it's working as designed).

Steps to reproduce

Technical details

• system: "x86_64-linux"
• host os: Linux 4.14.91, NixOS, 18.09.1834.9d608a6f592 (Jellyfish)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.1.3
• channels(sean): "nixos-19.03pre165037.eebd1a92637, nixpkgs-19.03pre165019.c06debd3f9d"
• channels(root): "nixos-18.09.1834.9d608a6f592"
• nixpkgs: /home/sean/.nix-defexpr/channels/nixpkgs