少し前に #swiftpm で話してましたが、SwiftPMでswift-driverを使うためですね。 https://github.com/apple/swift-package-manager/pull/2736 で--use-integrated-swift-driverオプションが追加されています。

[Build] Add support for building with the integrated Swift driver b...

Add bootstrap and build support for building the new Swift driver and importing it (as a library) into SwiftPM. Then introduce the new SwiftPM flag --use-integrated-swift-driver to enable use of th...