Overhaul packaging scripts and helpers:

- Shared helpers extracted to functions.sh for use by upstream packaging,
  Mod SDK, and downstream packaging (via the Makefile targets).
- Assembly management separated from data and combined between engine
  and mods to prepare for pending .NET core requirements.
- Streamline Makefile targets.
- Clean up a lot of old technical debt.

packaging/linux/openra-server.appimage.in

@@ -2,4 +2,4 @@
 HERE="$(dirname "$(readlink -f "${0}")")"
 cd "${HERE}/../lib/openra" || exit 1
 
-mono --debug OpenRA.Server.exe Game.Mod={MODID} "$@"
+mono --debug OpenRA.Server.exe Game.Mod="{MODID}" "$@"