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/configurations.nix

@@ -1,9 +1,31 @@
-{ config, ... }:
+{
+  config,
+  lib,
+  inputs,
+  ...
+}:
 let
-  inherit (cfg.lib) extractConfigurations;
+  inherit (lib) nixosSystem;
+  inherit (cfg.lib) flattenAttrs;
+  inherit (lib.attrsets) mapAttrs;
   cfg = config.flake;
   hosts = cfg.manifest.hosts or { };
+  mkConfigurations =
+    class: hosts:
+    mapAttrs (
+      _: value:
+      if class == "nixos" then
+        nixosSystem {
+          specialArgs = { inherit inputs; };
+          modules = [
+            (flattenAttrs cfg.modules.nixos)
+            (value.extraCfg or { })
+          ];
+        }
+      else
+        { }
+    ) hosts;
 in
 {
-  flake.nixosConfigurations = extractConfigurations "nixos/" hosts;
+  flake.nixosConfigurations = mkConfigurations "nixos" hosts.nixos;
 }

nix/lib/default.nix

@@ -1,59 +1,9 @@
-{
-  lib,
-  config,
-  inputs,
-  ...
-}:
+{ lib, ... }:
 let
-  cfg = config.flake;
-  inherit (lib.trivial) pipe;
-  inherit (lib.strings) removePrefix hasPrefix;
-  inherit (lib.attrsets)
-    concatMapAttrs
-    mapAttrs'
-    filterAttrs
-    mergeAttrsList
-    ;
+  inherit (lib.attrsets) concatMapAttrs;
 in
 {
-  flake.lib = rec {
+  flake.lib = {
     flattenAttrs = attrset: concatMapAttrs (_: v: v) attrset;
-    mkSystem =
-      prefix: name: value:
-      let
-        hostName = removePrefix prefix name;
-        hostConfig = value;
-        flakeConfig = config;
-        mkProfileCfg =
-          profileList: # List of attrsets of nixos configs
-          pipe profileList [
-            (map flattenAttrs) # List of nixos configs
-            mergeAttrsList
-          ];
-      in
-      {
-        name = hostName;
-        value = lib.nixosSystem {
-          specialArgs = {
-            inherit
-              inputs
-              hostName
-              hostConfig
-              flakeConfig
-              ;
-          };
-          modules = [
-            (flattenAttrs cfg.modules.nixos)
-            (mkProfileCfg (value.profiles or [ ]))
-            (value.extraCfg or { })
-          ];
-        };
-      };
-    extractConfigurations =
-      prefix: hosts:
-      pipe hosts [
-        (filterAttrs (name: _: hasPrefix prefix name))
-        (mapAttrs' (mkSystem prefix))
-      ];
   };
 }

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";