PyModules LCG_101 Update, master branch (2022.04.01.)

Tweaked PyModules to be compatible specifically with LCG_101. Instead of making configuration-time decision based on what externals are found, just tweaked the list to correspond to LCG_101. Which we'll have to update when we switch to a newer LCG version!

This is meant as a replacement of !936 (closed). @fwinkl, I just really don't like the idea of making the build behave differently in different environments. In the last year I had to realise through the Acts projects that that is just not a robust setup. There we went to a setup where the user can rigorously choose which stuff would be built by the project behind the scenes, and what it would expect to see "from the user" (using find_package). But whatever configuration the user chose, that's what the build would go with. (Or fail if the configuration was not appropriate for the build environment.)

Fixes ATLINFR-4466.