Swift: When possible, move Package.swift into the nested client directory

[cmgriffing]

Currently, SwiftPM requires the Package.swift to be at the root of the repo. This could get messy when supporting the 14 planned language SDKs we have for Vexilla.

It seems there is a forum post about this problem:
https://forums.swift.org/t/allow-package-swift-to-not-be-at-the-root-of-the-repository/60431/2

If SwiftPM solves this at some point, we should move the Package.swift file back into the nested folder