Swift 4 Migration

[youming-lin]

• SPM is changing executable location again. BasePath.project will be removed along with implicit Xcode support.
• Update Package.swift to Swift 4 format.
• Add new APIs to support Codable.

[svanimpe]

@youming-lin I am using BasePath.project to load project files. What should I use instead?

[youming-lin]

@svanimpe Depends on your use case, but I lean towards .pwd now. It is the method least affected by changes in build process. As it is currently,swift build --build-path can break .project as .project assumes the existence of a Package.swift somewhere in a parent folder of the executable.