Python / libffi Fixes, master branch (2022.03.28.)

Made sure that flake8_atlas and PyModules would use libffi correctly, fixing the issue described in SPI-2125.

Notice that:

• For PyAnalysis this was done a good while ago, in !779 (merged). But since that was a whole big thing, I didn't realise that I should set up libffi explicitly for these two additional packages as well.
• We set up libffi in exactly one place for Athena (https://acode-browser1.usatlas.bnl.gov/lxr/search?%21v=head&_filestring=CMakeLists.txt&_advanced=1&_string=libffi&_casesensitive=1). So the Athena runtime was actually using the correct library all along. It was the build of AthenaExternals that was sketchy so far.

Pinging @fwinkl.

P.S. I removed some REQUIRED keywords along the way. Since we should use built time errors instead of configuration errors in most of our builds. For the externals usually if anything breaks, everything breaks. But still, it should allow nightlies to at least progress a bit when something goes wrong.