nix: expose modular configuration combinators

nix/configurations/default.nix

@@ -1,11 +1,68 @@
 {
-  mkDarwins,
-  mkHomeManagers,
-  mkNixOSes,
+  self,
+  inputs,
+  many,
+  mkDarwin,
+  mkHomeManager,
+  mkNixOS,
   ...
 }:
+
+let
+  nixpkgsRegistry = {
+    # https://yusef.napora.org/blog/pinning-nixpkgs-flake/
+    # Has to be done here because hm-modules don't have access to flake inputs
+    nix.registry.nixpkgs.flake = inputs.nixpkgs;
+  };
+
+  mkNixOSes = many (
+    mkNixOS
+      (name: _: [
+        self.nixosModules._
+        self.nixosModules.layouts
+        ./host/${name}
+        inputs.agenix.nixosModules.default
+        inputs.home-manager.nixosModules.home-manager
+      ])
+      (
+        name: _: [
+          self.homeModules._
+          ./home/${name}
+          nixpkgsRegistry
+        ]
+      )
+  );
+  mkDarwins = many (
+    mkDarwin
+      (name: sys: [
+        { nixpkgs.hostPlatform = sys; }
+        self.nixosModules._
+        self.darwinModules._
+        ./host/${name}
+        inputs.home-manager.darwinModules.home-manager
+      ])
+      (
+        name: _: [
+          self.homeModules._
+          ./home/${name}
+          nixpkgsRegistry
+        ]
+      )
+  );
+  mkHomeManagers = many (
+    mkHomeManager (
+      name: _: [
+        self.homeModules._
+        ./home/${name}
+        nixpkgsRegistry
+        self.homeModules.auto-gc # Enable user gc only when home-manager is used standalone
+      ]
+    )
+  );
+in
+
 {
-  imports = [ ./combinators.nix ];
+  imports = [ ./combinators ];
 
   flake = {
     darwinConfigurations = mkDarwins {