ART Externals Fix, master branch (2018.07.19.)

These are packages that all used to live within pytools/, but by now SFT started removing them, and putting them into their individual packages. While urllib3 is still part of pytools as well in LCG_93x, it seemed safer to introduce Findurllib3.cmake at the same time.

Unfortunately the python packages in LCG releases are a bit of a mess. 😦 There are many packages that appear multiple times inside of a single LCG release. Hopefully all those multiple appearances are always for the same package version, but this is just a hope on my part...

Pinging @obreshko and @tcuhadar on this. The idea is that since Findpytools.cmake will take care of calling these new modules by itself, when we update the master branch to include this update, that should fix up the environment. (It did do that in my local tests...)

Also tagging @ivivarel for information.