Issue Description:

This tracks implementing the SE-0152 Package Manager Tools Version proposal.

Proposal is here: https://github.com/apple/swift-evolution/blob/master/proposals/0152-package-manager-tools-version.md

This proposal introduces a "Swift tools version" which is declared for each Swift package. The tools version declares the minimum version of the Swift tools required to use the package, determines what version of the PackageDescription API should be used in the Package.swift manifest, and determines which Swift language compatibility version should be used to parse the Package.swift manifest.

This feature shall be added to Swift 3.1, to allow packages to manage the transition from Swift 3 to Swift 4 compatibility.