Compatibility of Plugins

I don't think there's an obvious fix is compatibility of plugins. We currently build a package to determine compatibility. I'm not actually sure what happens if you run Swift build on a pure plugin package. There are always going to be things there that we might not be able to represent to people who are looking for a package. And so there's a limit to how far we can really go down that path, or at least how far we could go without making things right.