JSON Build Improvements, master branch (2020.06.10.)

This is meant to clean up the issue reported by @cheidegg, that IOVDbSvcLib is not set up correctly.

While I was at it, I thought I would do a bit more than the bare minimum to clean this up. So I refurbished the configuration of the two packages that use nlohmann_json directly at the moment. As they were both doing some questionable stuff.

Along the way I also had to find that the atlas_add_test(...) function still had two imperfections, which prevented me from implementing these updates in the way that I wanted. That's why this is a WIP for now. It should be enabled once we updated to using a tag of atlasexternals that already holds atlasexternals!687 (merged). (The externals update will happen in a separate MR. I'm only opening this one not to forget about these updates.)

Pinging @sroe as well.