nixos-modules/host: convert to directory

preparation for a splitup into multiple files

Author: astro

doc/src/host.md

@@ -52,5 +52,5 @@ module directly in your NixOS configuration:
 ```nix
 imports = [ (builtins.fetchGit {
   url = "https://github.com/astro/microvm.nix";
-} + "/nixos-modules/host.nix") ];
+} + "/nixos-modules/host") ];
 ```

flake.nix

@@ -147,7 +147,7 @@
 
         nixosModules = {
           microvm = import ./nixos-modules/microvm;
-          host = import ./nixos-modules/host.nix;
+          host = import ./nixos-modules/host;
         };
 
         defaultTemplate = self.templates.microvm;