Always generate a ROOT modulemap and install it.

To use C++ modules during runtime we need to generate a modulemap
and install it alongside the ROOT headers. However, right now
we need to turn on cxxmodules to generate a modulemap, which would
force experiments to fulfill all the depndencies that cxxmodules
brings with it (that is, a modern clang that can build ROOT with C++
modules).

This patch untangles the modulemap generation from the cxxmodules
option, so that we always generate a modulemap even when cxxmodules
is turned off. We now also install the modulemap alongside
the ROOT headers.

No functional change for normal ROOT expected here, as the modulemap
will just be ignored without having runtime C++ modules enabled.