[cxxmodules] Also install the overlay file and modulemaps

ROOT needs an overlay files in the include directory during runtime
to correctly mount the system modulemaps for libc/STL. For this
we need to generate and install a new overlay file that points to
the *installed* modulemaps for those libraries.

We can't use the existing modulemap as this one still points to the
original build directory.

We also need to install the related stl/lib modulemaps.