swift-package-manager

Add methods to return path components as an array of strings. It's likely that we'll want to optimize this to instead return an iterator that can return a slice at a time based on the path separator rules, so that we can avoid allocating everything up-front.

Most immediately, these methods are needed by the logic to make a
relative path from two absolute paths, which will be needed before
we can adopt the AbsolutePath and RelativePath classes in SwiftPM.