pkgs: Add _type = "pkgs"

(cherry picked from commit ad1e2500ef)
(cherry picked from commit aec730a0af)

pkgs/development/libraries/isl/0.14.1.nix

@@ -4,7 +4,10 @@ stdenv.mkDerivation rec {
   name = "isl-0.14.1";
 
   src = fetchurl {
-    url = "http://isl.gforge.inria.fr/${name}.tar.xz";
+    urls = [
+      "mirror://sourceforge/libisl/${name}.tar.xz"
+      "https://libisl.sourceforge.io/${name}.tar.xz"
+    ];
     sha256 = "0xa6xagah5rywkywn19rzvbvhfvkmylhcxr6z9z7bz29cpiwk0l8";
   };
 

pkgs/development/libraries/isl/0.17.1.nix

@@ -4,7 +4,10 @@ stdenv.mkDerivation rec {
   name = "isl-0.17.1";
 
   src = fetchurl {
-    url = "http://isl.gforge.inria.fr/${name}.tar.xz";
+    urls = [
+      "mirror://sourceforge/libisl/${name}.tar.xz"
+      "https://libisl.sourceforge.io/${name}.tar.xz"
+    ];
     sha256 = "be152e5c816b477594f4c6194b5666d8129f3a27702756ae9ff60346a8731647";
   };
 

pkgs/development/libraries/isl/0.20.0.nix

@@ -4,7 +4,11 @@ stdenv.mkDerivation rec {
   name = "isl-0.20";
 
   src = fetchurl {
-    url = "http://isl.gforge.inria.fr/${name}.tar.xz";
+    urls = [
+      "mirror://sourceforge/libisl/${name}.tar.xz"
+      "https://libisl.sourceforge.io/${name}.tar.xz"
+    ];
+
     sha256 = "1akpgq0rbqbah5517blg2zlnfvjxfcl9cjrfc75nbcx5p2gnlnd5";
   };
 

pkgs/top-level/all-packages.nix

@@ -24,6 +24,21 @@ let
 
 in
 {
+  # 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