util: introduce datapath for getting the location of supporting data files templates, help and locale data is normally stored as sub folders in the directory containing the source of the mercurial module. In a frozen build they live as sub folders next to 'hg.exe' and 'library.zip'. These different kind of data were handled in different ways. Unify that by introducing util.datapath. The value is computed from the environment and is always used, so we just calculate the value on module load.