The `applePackageMapping` function will allow us to keep expressions for
the newer releases separate from the originals where necessary, without
needing to change `generate-sdk-packages.sh`. If we ever want to support
multiple versions of source releases simultaneously we should consider
changing that to point to versioned subdirectories of all the packages.

Many of the source releases introduced here will need changes to the
expression to build successfully. These will be introduced one by one.

Releases with dependencies on other releases, e.g., libresolv depends on
Libinfo, mDNSResponder and configdHeaders, were getting those releases
from pkgs.darwin rather than pkgs.darwin."10.13.6". By making the scope
of callPackage overridable we can manipulate which packages are used to
supply dependencies.

These are not part of the SDK so we shouldn't need them in general.

TODO: Evaluate whether CF still needs these and add a privateHeaders
      output.