In cmake macro file use CMAKE_CURRENT_LIST_DIR consequently instead of mixed use with KF5I18n_DIR

Summary:
On runtime KF5I18n_DIR points to the same path as CMAKE_CURRENT_LIST_DIR,
but adds an additional hidden and obsolate level of indirection.

Test Plan: compiled and tested with client package

Reviewers: ilic, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: cgiboudeaux, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13406