related to https://github.com/NixOS/rfcs/pull/70

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.

The touchpad is painful to use unless the "psmouse" kernel module
is disabled.

Following the current instructions yields these errors for some reason:

```
$ sudo nixos-rebuild switch
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I), at (string):1:13
building Nix...
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I)
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos/modules/installer/tools/nix-fallback-paths.nix' was not found in the Nix search path (add it using $NIX_PATH or -I)
/tmp/nixos-rebuild.vs4JVw/nix
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)
building the system configuration...
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I)
```

The E495 and T495 are based on the same core Ryzen platform, so the
E495 tweaks apply similarly to the T495.

Tested on my own T495, works well.

The new test runner will evaluate all test profiles from the README.md
in parallel in separate nix-build processes.
Since we do not load all processes into one process,
this also helps saving memory.

This reverts commit 77983d16.

apparently github can link to wiki text fine.

Channel branches are now provided in the nixpkgs repo.

Issue #71176.

(Current XPS 15 works well)

At first (till third glance) I thought only the listed profiles are supported

And reorder other XPS 13.

This is pretty much a copy of XPS 9370, without kaby-laky and the throttle bug. I was getting lockups with the kaby-laky changes and the throttle bug did not seem to have an impact either way.