pkgs: Add _type = "pkgs"

pkgs/top-level/all-packages.nix

@@ -11,6 +11,21 @@ res: pkgs: super:
 with pkgs;
 
 {
+  # A module system style type tag
+  #
+  # Allows the nixpkgs fixpoint, usually known as `pkgs`
+  # to be distinguished nominally.
+  #
+  #     pkgs._type == "pkgs"
+  #     pkgs.pkgsStatic._type == "pkgs"
+  #
+  # Design note:
+  # While earlier stages of nixpkgs fixpoint construction
+  # are supertypes of this stage, they're generally not
+  # usable in places where a `pkgs` is expected.
+  # (earlier stages being the various `super` variables
+  #  that precede all-packages.nix)
+  _type = "pkgs";
 
   # A stdenv capable of building 32-bit binaries.  On x86_64-linux,
   # it uses GCC compiled with multilib support; on i686-linux, it's