nixos-hardware: prepare for merge by moving to nixos/hardware

It is going to be merged into nixpkgs, under `nixos/hardware`.

This will give better discoverability, and considering the low
turnover (less than 100 commits in the last year and only 350 total)
it won’t make any dent on the size of nixpkgs.

We have a monorepo, let’s use it.