Update Extra Module build steps in build_wrapper.py

Improve Extra Module build logic a bit. Also make the argument
validation more robust.

Extra module builds should depend only from Qt minor version.
So also create symlink for latest minor qt package to which
the Extra Module builds can refer.

Minor cleanups related to above changes as well.

Change-Id: If210543698853b411c72516255824e2b4a6e8d80
Reviewed-by: Antti Kokko <antti.kokko@digia.com>
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Matti Paaso <matti.paaso@digia.com>