Use carthage xcframeworks

[yonaskolb]

This enables XcodeGen to look up the xcframeworks that Carthage 0.37 builds. This PR is still a work in progress, but put up for reference. Resolves #1006

For now it requires adding carthageXCFrameworks: true to the spec options

[CraigSiemens]

I tried this out with our project and an issue I'm noticing is that since carthage supports getting precompiled frameworks, it's possible to get a mix of xcframeworks and frameworks. This flag seems to assume everything will be available as a xcframework.

As far as I can tell, there isn't a great way to know whether a dependency will be a xcframework when the project is generated. The only ways I can think of would be:

• Use the currently added flag as a global default, allow each dependency to override that setting as needed with an isXCFramework: {true|false} or type option.
• At project generation time, look in the Carthage/Build folder to see whether it's a xcframework or a framework
  • May slow down the project generation a bit
  • Would require that the carthage dependencies are build before the project is generated, which isn't needed currently.

[lsh-silpion]

Hi there, is there any progress here?