refactor: update imports to use glue/ and variants/

- flake.nix: import glue/fleet.nix instead of hosts/
- installer/artifacts.nix: use 'fleet' parameter instead of 'hosts'
- installer/modules.nix: auto-import from variants/ directory

installer/artifacts.nix

@@ -1,6 +1,6 @@
 {
   inputs,
-  hosts,
+  fleet,
   self,
   system,
 }:
@@ -45,7 +45,7 @@ let
     nixos-generators.nixosGenerate {
       inherit system;
       specialArgs = { inherit inputs; };
-      modules = hosts.modules.${hostName} ++ [
+      modules = fleet.modules.${hostName} ++ [
         {
           disko.enableConfig = lib.mkForce false;
           services.upower.enable = lib.mkForce false;
@@ -61,7 +61,7 @@ let
     nixpkgs.lib.nixosSystem {
       inherit system;
       specialArgs = { inherit inputs; };
-      modules = hosts.modules.${hostName} ++ [
+      modules = fleet.modules.${hostName} ++ [
         "${nixpkgs}/nixos/modules/installer/netboot/netboot.nix"
         {
           disko.enableConfig = lib.mkForce false;
@@ -70,14 +70,14 @@ let
       ];
     };
 
-  hostNames = builtins.attrNames hosts.nixosConfigurations;
+  hostNames = builtins.attrNames fleet.nixosConfigurations;
 
   # Generate installer ISOs for hosts that have "installer-iso" in their buildMethods
   installerPackages = lib.listToAttrs (
     lib.concatMap (
       name:
       let
-        cfg = hosts.nixosConfigurations.${name};
+        cfg = fleet.nixosConfigurations.${name};
       in
       if lib.elem "installer-iso" cfg.config.athenix.host.buildMethods then
         [
@@ -96,7 +96,7 @@ let
     lib.concatMap (
       name:
       let
-        cfg = hosts.nixosConfigurations.${name};
+        cfg = fleet.nixosConfigurations.${name};
       in
       if lib.elem "iso" cfg.config.athenix.host.buildMethods then
         [
@@ -115,7 +115,7 @@ let
     lib.concatMap (
       name:
       let
-        cfg = hosts.nixosConfigurations.${name};
+        cfg = fleet.nixosConfigurations.${name};
       in
       if lib.elem "ipxe" cfg.config.athenix.host.buildMethods then
         [
@@ -145,7 +145,7 @@ let
     lib.concatMap (
       name:
       let
-        cfg = hosts.nixosConfigurations.${name};
+        cfg = fleet.nixosConfigurations.${name};
       in
       if lib.elem "lxc" cfg.config.athenix.host.buildMethods then
         [
@@ -164,7 +164,7 @@ let
     lib.concatMap (
       name:
       let
-        cfg = hosts.nixosConfigurations.${name};
+        cfg = fleet.nixosConfigurations.${name};
       in
       if lib.elem "proxmox" cfg.config.athenix.host.buildMethods then
         [