Move EventInfo overlay to xAODEventInfoCnv

As discussed with @akraszna a while ago xAODEventInfoCnv is a better home for the xAOD::EventInfo overlay algorithm.

This MR also:

• updates the new-style configuration a bit adding detection when the old EventInfo conversion is needed and RDO_SGNL output (currently also saving the standard xAOD::EventInfo as this is default)
• enabling flake8 for the package
• adding tests for EventInfoCnvAlg and making sure that the pileup output key is initialised with false if empty

/cc @jchapman @ssnyder