Scanning the dependencies for kdelibs/khtml/ (when all files are scanned) is
now down to 4.6 s from 6.3 s before this change (without the
headerLocationCache it takes about 14 s here).
It doesn't really make sense to include the complete include path as part
of the key for the map, since the include path will be the same for
all files in a project, so it doesn't add anything.

Alex