nix: separate into multiple overlays and expose them from top-level

default.nix

@@ -0,0 +1,14 @@
+# It should return a set of nix derivations
+# and optionally the special attributes `lib`, `modules` and `overlays`.
+# It should NOT import <nixpkgs>. Instead, you should take pkgs as an argument.
+# Having pkgs default to <nixpkgs> is fine though, and it lets you use short
+# commands such as:
+#     nix-build -A mypackage
+
+{ pkgs ? import <nixpkgs> {} }:
+
+
+{
+  overlays = import ./nix/overlays;
+  pkgs = import ./nix/pkgs { inherit pkgs; };
+}