scan the whole install folder for qml dependencies

Summary:
set qml-root-path as the root install folder
of the application, as is used to scan for
import dependencies, and both qml files in share should be scanned
as well as other qml imports that may be installed in /lib

Test Plan: kirigami gallery deployment has again all the needed dependencies

Reviewers: apol

Reviewed By: apol

Subscribers: Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D6103