External Finding Update, 1.0 branch (2018.02.28.)
This is a pretty big update/cleanup of the find-modules, which was triggered by @dcasper's complaint, that he was having issues with building Athena from CernVM.
I now updated the lcg_external_module(...) function to disallow any external to be found under /usr if a variable pointing at the external (from setting up an LCG release) exists. This also meant removing the module specific protections from a few of the modules that had to be protected like this even for lxplus builds.
At the same time I finally moved many of the find-modules out of the AtlasCMake directory, into the individual packages under External/.
Finally I removed a few find-modules that we have only had in here for historic reasons. We'll have to stop using these modules when we include this latest update into 21.X. (Some we can stop using completely, for others we have better alternatives that we should use instead.)