[cxxmodules] Do not register optional modulemaps.
[cxxmodules] Do not register optional modulemaps.

For some common libraries we provide modulemaps such as tinyxml2 and boost.
Independently whether implicit or fno-implicit-module-maps is selected we insert
an overlay and register the modulemap directly.

This patch disables this when fno-implicit-module-maps is chosen. This should
resolve ROOT-10803 introduced by root-project/root#5709

olifre/root,karies/root,olifre/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,karies/root,karies/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root