`carthage archive` should produce a more human-friendly archive structure

The current carthage archive output produces a zip with a good name, but the actual contents of the zip use the Carthage/Build/$plat/... format. This is not very friendly to anyone who downloads the zipfile manually, because expanding it will give them a folder named Carthage.

Since Carthage itself doesn't actually care about the directory structure of the zip, we should instead change it to the following format:

FrameworkName/
    Mac/
        FrameworkName.framework
        FrameworkName.framework.dSYM
    iOS/
        FrameworkName.framework
        FrameworkName.framework.dSYM
        // bcsymbolmaps...
    tvOS/
        ...