At the same time, simplify CMakeLists magic for libraries that want to
include Swift code in the library. The Lib-less name of the library is
now always the module name for the library with any Swift additions,
extensions, etc. All vfs overlays now live in a common location to make
finding them easier from CMake functions. A new pattern is needed for
the Lib-less modules to re-export their Cxx counterparts.
In theory the clang module map should not have absolute paths for the
headers. Other Swift projects seem to use the -ivfsoverlay feature of
clang to work around this, but it seems difficult to get to work.