move sourcemod stuff to it's own directory

nix/overlay.nix

@@ -1,16 +1,5 @@
-final: prev: {
-  hl2sdk = (import ./hl2sdk.nix) {
-    inherit (final) fetchFromGitHub lib;
-    inherit (final.stdenvNoCC) mkDerivation;
-  };
-  ambuild = final.python3Packages.callPackage ./ambuild.nix {};
-  metamod-source = final.callPackage ./metamod-source.nix {};
-  sourcemod = final.callPackage ./sourcemod.nix {};
-  sourcepawn = final.callPackage ./sourcepawn.nix {};
-  sourcemod-includes = final.callPackage ./includes/sourcemod-includes.nix {};
-  buildSourcePawnScript = final.callPackage ./build-sourcepawn-script.nix {};
-  sourcemod-include-library = final.callPackage ./includes/sourcemod-include-library.nix {};
-  sourcemod-include-curl = final.callPackage ./includes/curl.nix {};
-  sourcemod-include-steamworks = final.callPackage ./includes/steamworks.nix {};
+final: prev:
+{
   srcds-tf2 = final.callPackage ./srcds {};
 }
+// (import ./sourcemod/overlay.nix) final prev

nix/sourcemod/overlay.nix

@@ -0,0 +1,15 @@
+final: prev: {
+  hl2sdk = (import ./hl2sdk.nix) {
+    inherit (final) fetchFromGitHub lib;
+    inherit (final.stdenvNoCC) mkDerivation;
+  };
+  ambuild = final.python3Packages.callPackage ./ambuild.nix {};
+  metamod-source = final.callPackage ./metamod-source.nix {};
+  sourcemod = final.callPackage ./sourcemod.nix {};
+  sourcepawn = final.callPackage ./sourcepawn.nix {};
+  sourcemod-includes = final.callPackage ./includes/sourcemod-includes.nix {};
+  buildSourcePawnScript = final.callPackage ./build-sourcepawn-script.nix {};
+  sourcemod-include-library = final.callPackage ./includes/sourcemod-include-library.nix {};
+  sourcemod-include-curl = final.callPackage ./includes/curl.nix {};
+  sourcemod-include-steamworks = final.callPackage ./includes/steamworks.nix {};
+}