Add a clang module map for libarchive

Add a clang module map for libarchive

When compiling libarchive using clang in module mode a special
module.modulemap file describes the structure of the header files
so that they can be imported modularly. Having this file makes
it easier for modular uses of the library out of the box so that
clients don't need to write their own, potentially making errors
in doing so.

Add a module.modulemap in the public header file location so that
clang and related tools can find it easily.