VecGeom has headers organised into a hierarchical structure for both
build and install, but their naming does not include reference to the
project. Thus on install, one can end up with a folder structure:

/usr/local/include/base
/usr/local/include/backend
...

As the names are very general, there is a potential for clashes both at
install and use time. The structure is also inconsisent with other
VecXXX projects such as VecCore, which installs it headers under a
folder named for the project, e.g.

/usr/local/include/VecCore/

Solve these issues by moving all VecGeom public headers folders under
a VecGeom folder. Update #include directives in VecGeom headers to
prefix the new folder name to internal includes. Update install()
command for headers to install the VecGeom folder.