kde-cmake: Add CMAKE_{MODULE,PREFIX}_PATH consistently.

Prior to this change, CMAKE_PREFIX_PATH was consistently set (which is
preferred for most find_package() calls) but CMAKE_MODULE_PATH was only
set for Qt5 (if Qt had a different prefix than the module being
installed).

This could lead to situations where "simplified" find_package() calls
would use "Module" mode (using CMAKE_MODULE_PATH) and end up falling
back to a search of system directories. This could result in finding the
wrong module if the module being searched for was installed on the
system.