Issue Description:

When a dependency has a prerelease identifier, for instance version 1.2.3-beta5, swiftpm is unable to build it. I've been able to trace it to the initializer in dep/Package.swift, which uses the hyphen to split the folder name and then assumes the last component in the version. For example, with version 1.2.3-beta5 of the package deck-of-playing-cards, the package would get checked out into the folder deck-of-playing-cards-1.2.3-beta5. That initializer then interprets the version as beta5 and refuses to build the package because the version is incorrectly formatted.