ART Externals Fix, 1.0 branch (2018.07.20.)

This is a sibling of !324 (merged).

Just like in that MR, Findpytools.cmake was taught to look for chardet, idna and urllib3. But unlike in the master branch, not finding those externals is not treated as an error. Since in LCG_87 and 88 those packages don't exist yet. Those externals are provided by the pytools package itself in those older LCG releases.

This behaviour should enable both AthAnalysis (based on LCG_93 at the moment) and all the other projects (based on either LCG_87 or LCG_88) to set up a correct runtime environment (for ART).

Pinging @tcuhadar for her info...