[cxxmodules] Allow ROOT_ prefix module to be implicitly build by rootcling

Previously, [system] was used to mark modules which were supporsed be
implicitly built by rootcling for runtime modules. This was a workaround, and it was causing
SourceLocation error in cxxmodules as it is also using the same
modulemap.

To remove the workaround, we are introducing a new convention that modules
start with ROOT_ is treated as "ROOT system modules", which can be build
implicitly.