Externals and Analysis Updates, master branch (2020.09.25.)

This is a pretty loaded merge request...

It updates all projects to atlasexternals-2.0.79. The list of changes wrt. atlasexternals-2.0.78 are the following (atlasexternals@2.0.78...2.0.79):

• Updated FindSYCL.cmake for better compatibility with the latest oneAPI versions;
• Updated FindUUID.cmake to set up a correct RPM dependency in case the LCG release provides this package (which it has not done for a while now);
  • Updated the build of yampl to pick up libuuid from the LCG release, if it is provided by the LCG release (should not affect the build in master);
• Updated the build of HDF5 to work correctly using Xcode 12.0 on macOS Catalina;
• Updated the build of ROOT to be more robust in using the Python version built as part of the same project as it is (should not affect the x86_64-centos7-gcc8-opt builds).

On top of these I also decided to make a few updates in this repository:

• After the failed attempt in !36360 (merged), I'm trying to clean up the build configuration of Control/RootUtils once more. Let's see what the CI will have to say about it. (The update is necessary to be able to build AnalysisBase on Ubuntu and macOS platforms.)
• After the discussion in !36641 (merged), I decided to fix the test in PhotonVertexSelection myself. Fixing ATLASG-1553.
  • I also added the package to the Athena project after fixing its test for that project as well.
• Fixed the standalone tests of TriggerAnalysisAlgorithms. Which were tweaked, but not actually fixed, in !36646 (merged).

The small change in AthenaPoolCnvSvc was needed to make the test in PhotonVertexSelection work. But since that was an actual shortcoming in that package, I decided to just fix it outright instead of working around it in the test.

Pinging @kpachal, @krumnack.