refactor(ops): Split //ops/nixos into different locations

Splits //ops/nixos into:

* //ops/nixos.nix - utility functions for building systems
* //ops/machines - shared machine definitions (read by readTree)
* //ops/modules - shared NixOS modules (skipped by readTree)

This simplifies working with the configuration fixpoint in whitby, and
is overall a bit more in line with how NixOS systems in user folders
currently work.

Change-Id: I1322ec5cc76c0207c099c05d44828a3df0b3ffc1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2931
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: glittershark <grfn@gws.fyi>

ops/nixos/www/cs.tvl.fyi.nix

@@ -1,31 +0,0 @@
-{ config, ... }:
-
-{
-  imports = [
-    ./base.nix
-  ];
-
-  config = {
-    services.nginx.virtualHosts."cs.tvl.fyi" = {
-      serverName = "cs.tvl.fyi";
-      serverAliases = [ "cs.tvl.su" ];
-      enableACME = true;
-      forceSSL = true;
-
-      extraConfig = ''
-        location = / {
-          return 301 https://cs.tvl.fyi/depot;
-        }
-
-        location / {
-          proxy_set_header X-Sg-Auth "Anonymous";
-          proxy_pass http://localhost:${toString config.services.depot.sourcegraph.port};
-        }
-
-        location /users/Anonymous/settings {
-          return 301 https://cs.tvl.fyi;
-        }
-      '';
-    };
-  };
-}