I'm not going to say it's impossible, but finding a subset of Darwin that could be split out into its own module is probably beyond the scope of this effort, and would involve liaising with other teams at Apple.* We should stick to something that just requires changes in Swift.
* Swift doesn't really handle Clang submodules; it just papers over them, and fails to do so for any modules with an overlay. So we really would need a separate top-level module.