CMake: systematically use the feature-summary

There's not much point in having a feature summary that will trip over just-a-few of the required packages, while also using REQUIRED in find_package() calls -- then you have to re-run CMake for all the REQUIRED ones you're missing, and then one more time for the packages that are required in the feature summary.

Use the feature summary (e.g. TYPE REQUIRED) consistently. Then you can run CMake once and learn about all the missing dependencies in one go.