Created by: robbertvanginkel
When an apple_library is marked as modular, add a modulemap to the generated headersymlinktree so other rules can import this library modularly.
Modules are always platform specific so we need to only have one platform specific headersymlinktree with a modulemap. We cannot forward the metadata to a CxxLibraryDescription as it will create two trees (platform specific and platform agnostic). This PR makes sure that if a library is modular, AppleLibraryDescription doesn't forward the CxxLibraryDescription.MetadataType.CXX_PREPROCESSOR_INPUT to a CxxLibraryDelegate but instead creates it itself.