`formatted_help()` introduced the override keyword, which means that the other two methods that were marked as virtual instead raised a warning about inconsistent use of override. This fixes that by bringing the synthprop (more) in line with the rest of the code-base.
Give formal index a proper title.
Use `Pass::formatted_help()` to assign the group, but still return `false` because the help text still comes from `Pass::help()`.
Tidy up some of the affected files' includes to make use of the shared `yosys.h` includes.
