Swift modulemap tree structure resolution

Question

I want to create a modulemap for a Swift iOS framework. It will be a wrapper around a c library. The problem is that the c library includes a file that is inside of another directory. For example "lib.h" has a following line #include "directory/function.h". But of course it can't be found. How can I solve it? The names are made up, but the problem is real. I will appreciate any answer, thanks.

Example structure

SwiftFramework.h
module.modulemap
clib
  lib.h
  directory
    function.h

// module.modulemap file content

framework module SwiftFramework {
    umbrella header "SwiftFramework.h"

    module CLib [extern_c] {
        header "lib.h"
    }
}

// lib.h file content

#include "directory/function.h"