Created by: robbertvanginkel
This adds a new SYMLINK_TREE_WITH_MODULEMAP on HeaderMode. This will be the headermode for when an apple_library is modular.
The modulename is derived from the links, if any links are present. Lets the new mode integrate with the currently available interfaces of CxxPreprocessables. It should be okay as a modulemap will only work if the headers are in the Apple style.