FindAlugrid.cmake: do not try to include the whole grid

first this makes the test very slow, second (and more importantly) it
makes the test fail on dune master as the header relies on the
presence of the DUNE_GRID_VERSION* macros which are not present at the
configure stage of the build...
fix the dune 2.4 deprecation warnings

mainly this boils down to mapper.map(e) -> mapper.index(e)