CMake: Add a CLEAR_FEATURE macro and use it in PACKAGE_HANDLE

CMake: Add a CLEAR_FEATURE macro and use it in PACKAGE_HANDLE

Let's play it save and reset all "feature" variables (such as
<FEATURE>_INCLUDE_DIRS, <FEATURE>_LIBRARIES, etc.) before we populate
them in DEAL_II_PACKAGE_HANDLE(...).

This guards against issues where we include an external project
configuration that actually sets variables, such as <FEATURE>_CXX_FLAGS,
that we then accidentally use in our configuration even though this was
never intended.