create the whole plugin structure in the build directory

Summary:
when building copy all qml files in build/org/kde/kirigami.2
as well generating the binary plugin there.
this is neccessary to be able to load and test the whole QML
import without installing it system wide

Test Plan:
after make, the hierarchy under build is complete with the same files
that end up being installed

Reviewers: Kirigami, dfaure, sitter, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D7794