Due to the change of using json based plugin information instead
of xml based (pluginspec) this script became disfunctional long
time ago.. Adapt the script to use the json information generated
while building QC.
This script is still intended to find unused or unlisted
dependecies of plugins.

Prerequisites: Linux platform
Usage (just as before): inside the build directory of QC just run
the script directly or pass it to the Python interpreter.
(still limited to Python 2.x with x>=7)

Change-Id: I92b48327944df1239e9fac7fbd8ed1bee4fefa61
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>