Copy modules from NUR into NUR's flake

## Description

Right now, to use a NUR module from its flake, the path that has to be used is:

```nix
nur.legacyPackages."${system}".repos."${repo}".modules."${module}"
```

It would be great if the standard flake attributes could be used instead, like:

```nix
nur.nixosModules.repos."${repo}"."${module}"
```