nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/services/postgresql-backup/default.nix

@@ -1,28 +0,0 @@
-# Backup your data, kids!
-{ config, lib, ... }:
-let
-  cfg = config.my.services.postgresql-backup;
-in
-{
-  options.my.services.postgresql-backup = {
-    enable = lib.mkEnableOption "Backup SQL databases";
-  };
-
-  config = lib.mkIf cfg.enable {
-    services.postgresqlBackup = {
-      enable = true;
-      backupAll = true;
-      location = "/var/backup/postgresql";
-    };
-
-    my.services.backup = {
-      paths = [
-        config.services.postgresqlBackup.location
-      ];
-      # No need to store previous backups thanks to `restic`
-      exclude = [
-        (config.services.postgresqlBackup.location + "/*.prev.sql.gz")
-      ];
-    };
-  };
-}