Move declarativeplugin to top-level dir
Summary: It's not only used by the plasmoid any more but also SFOS and the app use it.
Test Plan: Builds, App runs fine
Reviewers: KDE Connect, piggz, apol
Reviewed By: KDE Connect, piggz, apol
Subscribers: piggz, kdeconnect
Tags: KDE Connect
Differential Revision: https://phabricator.kde.org/D14567