Compile package structure plugins into expected subdirectory

Summary: This means they're found by unit tests pre-installation

Test Plan:
Removed installed files
Ran make (but not make install)
ConfigModelTest now passes

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22682