Swift packages are normally in `Sources/<target-name>`, but
SwiftifyImportMacro is in `Sources/SwiftMacros`, and SwiftifyImport is
in `<root>/stdlib/public/core`, which is outside the package root. This
sets the correct path for SwiftifyImportMacro, and creates a directory
`Sources/PublicCorePortal` with a symlink to `SwiftifyImport.swift`.
It also enables LifetimeDependence for SwiftifyImport.
These changes are purely to provide information to SourceKit - this file
isn't actually used by CMake.