Detect qt sub projects and special case their build directory

Check every project's path against the source paths of all existing qts.
If we find a match, make it build in the right place by default.
Works for both in source and shadow builds.

Note: There's a quadratic algorithm since foreach kit we check against
all qt versions. That's unlikely to be a problem and non trivial to
fix.

Change-Id: I9f3456f3e835ee6adc35c26fe5c328c01387a8aa
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>