Continue to concentrate all the code reading random cmake files in
BuildDirManager. Now the task is to clean up the code, make it less
dependent on values it should not depend on (kits, etc.), make it
handle changes better and finally add another implementation that
uses the cmake server mode to extract the data.

Change-Id: I533625e376b969b64287bc205bd2e4be7a605306
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>