Filter file developments
These changes are supposed to tackle two issues:
- There were some complaints already that the package filtering rules are not re-read when someone runs the CMake configuration again in an existing build directory. The code was now updated to make sure that the filter file is re-read on every configuration.
- The filtering doesn't add a ".*" at the end of the paths specified in the filter file anymore. Instead it now adds a "$". Treating the path specified in the filter file as an exact one. To make it a bit easier to write correct filter files.