Summary:
This fixes issue reported by Klaus in Slack:
> when I change something in the plugin driver, I always have to rebuild the plugins before I can debug again.
Also refactored `libraries.pri` to remove duplication.
Reviewers: #swift_pilot_client, rwinklmeier
Reviewed By: #swift_pilot_client, rwinklmeier
Subscribers: kbasan, jenkins
Tags: #swift_pilot_client
Differential Revision: https://dev.swift-project.org/D31