This is effectively a rewrite of `overrideSDK`. It was required because
`wrapGAppsHook` propagates `depsTargetTarget` with the expectation that
it will effectively be `buildInputs` when the hook is itself used as a
`nativeBuildInput`. This propagates Gtk, which itself propagates the
default Dariwn SDK, making it effectively impossible to override the SDK
when a package depends on Gtk and uses `wrapGAppsHook`.
This rewrite implements the following improvements:
* Cross-compilation should be supported correctly (untested);
* Supports public and private frameworks;
* Supports SDK `libs`;
* Remaps instead of replacing extra (native) build inputs in the stdenv;
* Updates any Darwin framework references in `nix-support`; and
* It updates `xcodebuild` regardless of which input its in.
The implementation avoids recursion for performance reasons. Instead, it
enumerates transitive dependencies and walks the list from the leaf
packages backwards to the parent packages.
