darwin.stdenv: refactor stdenv definition

In preparation for bumping the LLVM used by Darwin, this change
refactors and reworks the stdenv build process. When it made sense,
existing behaviors were kept to avoid causing any unwanted breakage.
However, there are some differences. The reasoning and differences are
discussed below.

- Improved cycle times - Working on the Darwin stdenv was a tedious
  process because `allowedRequisites` determined what was allowed
  between stages. If you made a mistake, you might have to wait a
  considerable amount of time for the build to fail. Using assertions
  makes many errors fail at evaluation time and makes moving things
  around safer and easier to do.
- Decoupling from bootstrap tools - The stdenv build process builds as
  much as it can in the early stages to remove the requirement that the
  bootstrap tools need bumped in order to bump the stdenv itself. This
  should lower the barrier to updates and make it easier to bump in the
  future. It also allows changes to be made without requiring additional
  tools be added to the bootstrap tools.
- Patterned after the Linux stdenv - I tried to follow the patterns
  established in the Linux stdenv with adaptations made to Darwin’s
  needs. My hope is this makes the Darwin stdenv more approable for
  non-Darwin developers who made need to interact with it. It also
  allowed some of the hacks to be removed.
- Documentation - Comments were added explaining what was happening and
  why things were being done. This is particular important for some
  stages that might not be obvious (such as the sysctl stage).
- Cleanup - Converting the intermediate `allowedRequisites` to
  assertions revealed that many packages were being referenced that no
  longer exist or have been renamed. Removing them reduces clutter and
  should help make the stdenv bootstrap process be more understandable.

1 files changed, 1 insertions, 1 deletions

diff --git a/pkgs/stdenv/default.nix b/pkgs/stdenv/default.nix
index 7a2ad665e09..6cc1339752a 100644
--- a/pkgs/stdenv/default.nix
+++ b/pkgs/stdenv/default.nix
@@ -28,7 +28,7 @@ let
   # the GNU C compiler, and so on.
   stagesLinux = import ./linux args;
 
-  inherit (import ./darwin args) stagesDarwin;
+  stagesDarwin = import ./darwin args;
 
   stagesCross = import ./cross args;