Moved the FindFastJetContrib.cmake module to AtlasLCG, as we may
now use this external without building it ourselves. Updated the
script along the way to behave like an AtlasLCG module should.

At the same time, to increase our flexibility, taught the
FastJetContrib package itself how it should build itself in case
it is built against a FastJet version not being built by the same
project. To do this, the FindFastJet.cmake module in AtlasLCG had
to be made a little smarter as well.