refactor: move providers out of nested dir

README.md

···
 in your NixOS configuration. for example, `nixosModules.firewall` and
 `nixosModules.firewall-hetzner`. see `nix flake show` for all available modules.
 
-if not using flakes, you can import `<resource>/` and `<resource>/provider/<provider>`.
+if not using flakes, you can import `<resource>/` and `<resource>/<provider>`.
 
 then, you can either use each module's individual `mkApp` config option to
 generate an app and run it, or you can call `makeApps`:

flake.nix

···
   in {
     nixosModules = {
       firewall = ./firewall;
-      firewall-hetzner = ./firewall/provider/hetzner;
+      firewall-hetzner = ./firewall/hetzner;
     };
     checks =
       l.mapAttrs
       (_: pkgs: {
-        firewall-hetzner-app = import ./firewall/provider/hetzner/app.nix {
+        firewall-hetzner-app = import ./firewall/hetzner/app.nix {
           inherit pkgs;
           taggedPorts = {
             http.allowedTCPPorts = [80 443];