Add mixed_langauge_library

[brentleyjones]

Replaces experimental_mixed_langauge_library in rules_apple. Meant to support SE-0403.

[brentleyjones]

Since we make sure that #import <ModuleName/ModuleName-Swift.h> works (though I need to make that work for the _clang target as well I guess...), I think a header map can be made as a target before a mixed_language_target and passed in the same way as it would for a normal {objc,swift}_library target. If that's the case, rules_swift wouldn't need to mess with them at all.

[luispadron]

Thank you so much for starting the move over to rules_swift, this is looking great!

[brentleyjones]

There is a request to allow the macro to use non-standard rules for the clang and swift targets. So instead of swift_library one could use their own custom_swift_library. I think this can be done in a followup change, to prevent blocking the core functionality here.