Support producing xcframework that wraps static framework instead of static library

[keith]

Many 3rd party dependencies in the iOS ecosystem expect to be consumed as a framework and therefore setup their #imports in a way that expects that. Today if you build a static xcframework with rules_apple you get a xcframework that wraps a static library. I think we should also support this case to support the include syntax that most projects use, without having to modify the imports to make things work.