[cxxmodules] Use the global module index only when no rootmap candidate is found
[cxxmodules] Use the global module index only when no rootmap candidate is found

In the hybrid mode when we use ROOT with modules and third party software
without modules we have two name resolution systems: the global module index (GMI)
and the rootmaps. In case an identifier is defined in both (eg TMVA::Event and
X::Event), the GMI will take priority and load the TMVA module without later
allowing the system to look for other candidates.

This patch uses the GMI only when no rootmap file has told ROOT that it
provides names for the given namespace.

This fixes root-project/root#9583

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root