Previously I got the following error message:
```
error: opening file '/home/ma27/Projects/nixpkgs/nixos/modules/installer/default.nix': No such file or directory
```
Probably related to 6c68fbd4e1.
This reverts commit 10addad603, reversing
changes made to 7786575c6c.
NixOS scripts should be kept in the NixOS source tree, not in
pkgs. Moving them around is just confusing and creates unnecessary
code/history churn.
Move all the nixos-* scripts from the nixos distribution as real
packages in the pkgs/ package set.
This allows non-nixos users to run the script as well. For example,
deploying a remote machine with:
nixos-rebuild --target-host root@hostname --build-host root@hostname
