refactor: Tidy flake.nix and break it apart

outputs/homeConfigurations.nix

@@ -0,0 +1,26 @@
+{ ... }@inputs:
+
+let users = builtins.readDir ../users;
+    mkUser = username: _: {
+      imports = [
+        (import ../users/${username}/default.nix).homeConfiguration
+
+        inputs.self.homeManagerModules.default
+
+        inputs.sops-nix.homeManagerModules.sops
+        inputs.impermanence.homeManagerModules.impermanence
+
+        # Directory name should always match username.
+        ({ ... }: { home.username = username; })
+
+        ({ lib, ... }: {
+          nix = {
+            settings.experimental-features =
+              lib.mkDefault
+                [ "nix-command" "flakes" ];
+          };
+        })
+      ];
+    };
+in
+builtins.mapAttrs mkUser users

outputs/nixosConfigurations.nix

@@ -0,0 +1,59 @@
+{ nixpkgs, ... }@inputs:
+
+let
+  # Given a hostName, return a NixOS module defining each user specified by
+  # `sydnix.users.users`.
+  mkHostUsers = hostName: { config, lib, ... }: {
+    users.users =
+      (lib.mapAttrs
+        # Only import the user's system configuration.  The code responsible for
+        # importing the user's home configuration lives in
+        # outputs/homeConfigurations.
+        (username: _: (import ../users/${username}).systemConfiguration)
+        # The directory users/ hosts a 'universe' of user profiles, which
+        # specific hosts may select any subset of, by setting the option
+        # `sydnix.users.users` to a list of choice usernames.
+        (lib.filterAttrs
+          (username: _: builtins.elem username config.sydnix.users.users)
+          (builtins.readDir ../users)));
+  };
+
+  mkHost = hostName: nixpkgs.lib.nixosSystem {
+    specialArgs = inputs;
+    system = import ../hosts/${hostName}/system.nix;
+    # TODO: This is very ad-hoc, and I don't like it.  Organise this better.
+    modules = [
+      ../hosts/${hostName}/configuration.nix
+
+      inputs.self.nixosModules.default
+
+      inputs.disko.nixosModules.disko
+      inputs.sops-nix.nixosModules.sops
+      inputs.impermanence.nixosModules.impermanence
+
+      # Directory name should always match host name.
+      ({ ... }: { networking.hostName = hostName; })
+
+      (mkHostUsers hostName)
+
+      # home-manager configuration.
+      inputs.home-manager.nixosModules.home-manager
+      ({ config, lib, self, ... }: {
+        home-manager.useGlobalPkgs = true;
+
+        home-manager.users =
+          lib.filterAttrs
+            (username: _: builtins.elem username config.sydnix.users.users)
+            self.homeConfigurations;
+
+        home-manager.extraSpecialArgs = {
+          utils = import ../lib/utils.nix {
+            inherit config lib;
+            pkgs = nixpkgs;
+          };
+        };
+      })
+    ];
+  };
+in
+builtins.mapAttrs (dirName: _: mkHost dirName) (builtins.readDir ../hosts)