This commit introduces the `nixpkgs-disabled` module, that is
basically a mock of `nixpkgs` module where any value different from
`null` will cause an assertion error.
This is to help debugging cases where `home-manager.useGlobalPkgs` is
set to `true` and `nixpkgs.*` options are being used.
Nowadays this returns the following error:
```
error: The option `home-manager.users.<user>.nixpkgs` does not exist.
```
This will change too:
```
error: `nixpkgs` options are disabled when `home-manager.useGlobalPkgs` is enabled.
```
That will direct the user to the correct solution (either removing
`nixpkgs` or disable `home-manager.useGlobalPkgs`).
