One directory may contain many pro files. The generator was happily
generating the same CMakeLists.txt for all of them (overwriting).
This patch implements a different logic. It tries to find the main
pro file and skips others assuming that somehow implicitly they will
be incorporated (for example through SUBDIRS).
Change-Id: Ie07d75e900a96dd48bf981a896c9dfb920f39a23
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>