Example:
```
error: `self` (without `'`) is not a `perSystem` module argument, but a
module argument of the top level config.
The following is an example usage of `self`. Note that its binding
is in the `top` parameter list, which is declared by the top level module
rather than the `perSystem` module.
top@{ config, lib, self, ... }: {
perSystem = { config, self', ... }: {
# in scope here:
# - self
# - self'
# - config (of perSystem)
# - top.config (note the `top@` pattern)
};
}
```
