Move FlightGear to stable release 2018.3.5
Move FlightGear to latest stable release 2018.3.5 (apparently, 2019.1.1 was a development release, now unsupported).
Chagelog for 2018.3.x branch is available here.
In this package release assets were moved to flightgear-data package to ease future maintenance and rebuilds.
Test Plan: Set up a flight simulation and performed a flight.
Reviewers: Triage Team, DataDrake
Reviewed By: Triage Team, DataDrake
Differential Revision: https://dev.getsol.us/D8893