build: create modulemap symlinks
build: create modulemap symlinks

Use a darwin specific modulemap on darwin, and the generic one
elsewhere.  This fixes the cmake build to create the symlinks like the
autotools build and cause the installation to be correct on Darwin as
well as the build.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch