GitVersionPlugin

A SwiftPM plugin that codegens a version number, as read by Git, for use in tooling

Split from Apple's swift-testing repo and Xcode plugin added to the interface.

The version is determined from:

• If the repository is sitting at a tag with no uncommitted changes, use the tag.
• Otherwise, use the commit hash (with a "there are changes" marker if needed.)
• Finally, fall back to nil if nothing else is available.