lib/modules: Fix nonexistant option error
lib/modules: Fix nonexistant option error

The refactoring in https://github.com/NixOS/nixpkgs/commit/fd75dc876586bde8cdb683a6952a41132e8db166
introduced a mistake in the error message that doesn't show the full
context anymore. E.g. with this module:

  options.foo.bar = lib.mkOption {
    type = lib.types.submodule {
      baz = 10;
    };
    default = {};
  };

You'd get the error

  The option `baz' defined in `/home/infinisil/src/nixpkgs/config.nix' does not exist.

instead of the previous

  The option `foo.bar.baz' defined in `/home/infinisil/src/nixpkgs/config.nix' does not exist.

This commit undoes this regression

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs