xAODViews CMake Cleanup, master branch (2020.05.15.)

With !32081 (closed) finally concluding, I'm now splitting it into multiple independent and (hopefully) manageable MRs.

In this update I cleaned the build configuration of xAODViews by:

• Declaring an interface library (xAODViewsLib) in it;
• Removing its non-existent (direct) dependence on ROOT;
• Updating its dictionary (xAODViewsDict) to use the newly defined interface library.

The only client of xAODViews is/was xAODViewsCnv. Which didn't just have a horrific CMake configuration, but it doesn't/didn't even have the correct name. 😦 So now I:

• Renamed the package to xAODViewsAthenaPool, so that it would be in sync with the naming conventions of all of the other xAOD packages;
• Greatly simplified the package's build configuration with the help of the newly defined xAODViewsLib library.

Pinging @ssnyder for information purposes. 😉