refactor(nixos): simplify configurations.nix and lib

This commit refactors configurations.nix and lib/default.nix by removing the
extractConfigurations function and related code from lib/default.nix,
and moving the NixOS system building logic directly into configurations.nix.
It also changes the structure of hosts in manifest.nix to use a
hosts.nixos attribute.

nix/manifest.nix

@@ -1,3 +1,4 @@
+
 let
   testCfg =
     { hostName, ... }:
@@ -21,8 +22,8 @@ in
       shell = "fish";
       pubkey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILdsZyY3gu8IGB8MzMnLdh+ClDxQQ2RYG9rkeetIKq8n rafiq";
     };
-    hosts = {
-      "nixos/nemesis" = {
+    hosts.nixos = {
+      nemesis = {
         machine = {
           platform = "amd";
           gpu = "nvidia";
@@ -38,7 +39,7 @@ in
         };
         extraCfg = testCfg;
       };
-      "nixos/apollo" = {
+      apollo = {
         machine = {
           platform = "intel";
           root.drive = "/dev/disk/by-id/nvme-eui.002538d221b47b01";