make nixos module hydra from this repository by default

When people reach out to the git repository they probably want to use
hydra from the same source.
This also removes the need for an overlay with simpler and more
performant direct use of the nixpkgs passed in. Before it was
re-importing nixpkgs.

test

nixos-modules/default.nix

@@ -1,11 +1,10 @@
-{ overlays }:
+{ self }:
 
 {
-  hydra = import ./hydra.nix;
-
-  overlayNixpkgsForThisHydra = { pkgs, ... }: {
-    nixpkgs = { inherit overlays; };
-    services.hydra.package = pkgs.hydra;
+  hydra = { pkgs, lib,... }: {
+    _file = ./default.nix;
+    imports = [ ./hydra.nix ];
+    services.hydra-dev.package = lib.mkDefault self.packages.${pkgs.hostPlatform.system}.hydra;
   };
 
   hydraTest = { pkgs, ... }: {