lib: fix and simplify `doRename`

Before this change `mkRenamedOptionModule` would override option defaults
even when the old option name is left unused. For instance

```nix
{
  optios = {
    services.name.new = mkOption {
      default = { one = {}; };
    };
  };
  imports = [
    (mkRenamedOptionModule [ "services" "name" "old" ] [ "services" "name" "new" "two" ])
  ];
  config = {};
}
```

would evaluate to

`{ config.services.name.new = { two = {}; }; }`

when you'd expect it to evaluate to

`{ config.services.name.new = { one = {}; }; }`.