headers and sources paths should be canonicalized

The headers and sources configuration variables have values that are relative to the location of the qdocconf file they are contained in. Currently, these file paths are not canonicalized before they are added to the list of files to be parsed by qdoc. But the paths of files obtained from the headerdirs and sourcedirs variables are canonicalized. This difference can result in the same file appearing in the list twice, which makes qdoc parse it twice.