Build platform plugins in correct sub directory of build dir

Summary:
This brings KWin a step closer to be run from build dir without having
to install at all. The integration tests are adjusted so that the
virtual platform is still found which makes the code be closer to what
is used in normal kwin_wayland.

Test Plan: ctest passes, manually verified correct plugin is loaded

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17388