Clean-up of Geo2G4/CMakeLists.txt

A few dependencies of `Geo2G4Lib` were moved from being private to public.
The dependent component library and dictionary then only need to link against
`Geo2G4Lib`. NB It is necessary for `Geo2G4Lib` to link against `GeoPrimitives`
publicly as `GeoModelKernel` does not seem to provide the `Eigen` headers.