nixpkgs-bootstrap: rework "mkNixpkgs" such that "nixpkgs-bootstrap.$channel.updateScript" can be evaluated *without* importing the underlying nixpkgs

this makes it possible to update out of a nixpkgs staging rev whose toplevel attrset doesn't eval

impure.nix

@@ -10,7 +10,7 @@
 let
   mkNixpkgs = import ./pkgs/by-name/nixpkgs-bootstrap/mkNixpkgs.nix {};
   mkPkgs = branch: args: (
-    mkNixpkgs (args // { inherit branch; })
+    (mkNixpkgs (args // { inherit branch; })).pkgs
   ).extend (import ./overlays/all.nix);
   pkgs = mkPkgs "master" { inherit localSystem; };
   inherit (pkgs) lib;

pkgs/by-name/nixpkgs-bootstrap/mkNixpkgs.nix

@@ -91,17 +91,16 @@ let
         } // args');
       }
     else
-      # N.B.: this is crafted to allow `nixpkgs.FOO` from other nix code
-      # AND `nix-build -A nixpkgs`
       patchedSrc.overrideAttrs (base: {
         # attributes needed for update scripts
         inherit version;
         pname = "nixpkgs";
-        passthru = (base.passthru or {}) // nixpkgs // {
+        passthru = (base.passthru or {}) // {
           # override is used to configure hostPlatform higher up.
           override = overrideArgs: mkNixpkgs (args // overrideArgs);
 
-          # N.B.: src has to be specified in passthru, not the outer scope, so as to take precedence over the nixpkgs `src` package
+          pkgs = nixpkgs;
+
           src = {
             # required by unstableGitUpdater
             gitRepoUrl = "https://github.com/NixOS/nixpkgs.git";

pkgs/default.nix

@@ -54,9 +54,6 @@ let
     });
 
     ### aliases
-    # nixpkgs = nixpkgs-bootstrap.master;
-    # nixpkgs-staging = nixpkgs-bootstrap.staging;
-    # nixpkgs-next = nixpkgs-bootstrap.staging-next;
     inherit (trivial-builders)
       copyIntoOwnPackage
       deepLinkIntoOwnPackage