Adapt to core CMake and Eigen changes

This adapts to the changed CMake target names (acts-core!454 (merged)) and the removed Eigen plugin (acts-core!455 (merged)). Is rebased on top of !103 (merged), so that one needs to go in first.

This updates the submodules, but will probably need to be re-updated when we merge acts-core and acts-fatras (acts-fatras!13 and acts-fatras!14)

I cleaned up the Pythia8 plugin's directory structure a bit, it was using the same folder for cpp and hpp files.