top-level: Normalize stdenv booting

Introduce new abstraction, `stdenv/booter.nix` for composing bootstraping
stages, and use it everywhere for consistency. See that file for more doc.

Stdenvs besides Linux and Darwin are completely refactored to utilize this.
Those two, due to their size and complexity, are minimally edited for
easier reviewing.

No hashes should be changed.

pkgs/stdenv/darwin/make-bootstrap-tools.nix

@@ -334,8 +334,8 @@ in rec {
   # The ultimate test: bootstrap a whole stdenv from the tools specified above and get a package set out of it
   test-pkgs = import test-pkgspath {
     inherit system;
-    stdenvFunc = args: let
+    stdenvStages = args: let
         args' = args // { inherit bootstrapFiles; };
-      in (import (test-pkgspath + "/pkgs/stdenv/darwin") args').stdenvDarwin;
+      in (import (test-pkgspath + "/pkgs/stdenv/darwin") args').stagesDarwin;
   };
 }