Having either argument defined based on the OS is a problem when
trying to write generic Nix code.
The current workaround is to use accept both and specify a default
value for each argument:
```
{ config, lib, nixosConfig ? {}, darwinConfig ? {}, ... }:
let
osConfig = nixosConfig // darwinConfig;
in
{
# Do something with `osConfig`
}
```
With this commit, it becomes possible to do the following:
```
{ config, lib, osConfig, ... }:
{
# Do something with `osConfig`
}
```
60ebc273c9