The CbmData library depends on flesnet so we have to provide it. Add all code
needed to build an external package and the complete external directory at
once. Currently only flesnet package is build.

Copy the file from the FairRoot installation and add the FairRoot targets.
It make sense to have teh find_package module inside of CbmRoot.

Since the FairRoot project not yet installs a CMake configuration file which
contains all FairRoot targets we define them ourself.
When FairRoot provides the configuration file no additional should be needed.