This decreases the memory consumption for the defines/include paths
as queried by the CMake JSON import job considerably. And, since
we can now reuse existing Path instances, we don't need the costly
conversion most of the time, leading to a considerable performance
win as well.

Before, my kdevelop session took ca. ~230MB of memory, now this
number is down to ~153MB. Performance wise, I saw the kdevplatform
JSON file speed up from ~2.2s evaluation down to ~1.4s.