Refactor post-processing of generated build paths
This MR makes kf6-cmake a base class for package path cleanup as leaked by the build-system. This class is supposed to be also used in meta-kde with only the very basic required handlings. Functional wise the changes are:
- unify the different approaches for cmake absolute path cleanup for target files
- introduce cleanup for cpp/h files that contain comments with full-system paths for tool invocation and source files as created by rcc, dbusxml2cpp etc. Overall this makes the output in Nanbield much less noisy such that the actual problems can be seen.