Task:	229217

without that, the auto-included highly complex files from mkspecs/ make
it spew out oodles of warnings.
by default, the user-provided files are still scanned in "cumulative"
mode (ignoring conditionals), so we collect as many files as possible.
(ported from qt)

code mostly c&p-d from qmake (sync from qt).

use qset instead of qlist

(merge aa87b75910a99a26e7ec581aa38f8982fe231ad0 from qt)

Details:  ProFileCache be gone, as is ManagedProFile, not to be missed.
Drastically simplified how ownership works.