Do not use FeatureSummary to report found packages

The use of feature_summary in GaudiDependencies.cmake is a bad idea as it produces a lot of noise in downstream projects (see #181 (closed)).

This MR drops it to use the normal REQUIRED flag of find_package.

A better approach is to use find_dependency in GaudiConfig.cmake (see #205), but for the moment this is enough.

Closes #181 (closed)

/cc @fwinkl