This seems to be an implementation question and not an evolution one, so I'm going to change the category of this thread.
I'm really not sure what you're asking here, so I'll just give a quick technical summary. A swiftmodule file is a binary description of the interface to a Swift module. It contains binary serializations of the public and ABI-exposed declarations of the module, as well as binary serializations of the SIL representation of functions which can inlined outside of the module. It is not an object file and does not contain machine code, and therefore it cannot be linked.