make servicerunner only build once

Summary:
build the runner itself in a static lib we can use for testing and then
link that into a module lib together with the factory entry point to
compose the final lib for installation

means servicesrunner.cpp is now only built once, whereas before it was
built twice through 100% duplicative targets

Test Plan: test still passes, installing and restarting krunner still yields working service searches

Reviewers: broulik, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28137