Add support for SPM installation & usage

[s0m3chill]

As for now we can only use it via Cocoapods, please add Swift Package Manager

[ileitch]

You can use SwiftPM if you like, but it will result in a binary built with the Debug configuration, which is slow. SwiftPM doesn't yet support the ability to ship a pre-built binary with the entitlements that Periphery requires.