I suppose this is related to you Workspace proposal so I think we could go with something like this:
public static func package(localPath: String) -> Package.Dependency
public static func package(url: String, from version: Version) -> Package.Dependency
public static func package(url: String, from version: Version, localPath: Bool) -> Package.Dependency
With last version, if there is package in the local path Swift PM would use it, if not it would get it from the Git URL version. Also, I think the Workspace could be integrated in Package.swift.