gov efficiency tweaks

support method to convert PC vs AIDifficulty into efficiency rating
reordered govEvent to more logical AI awareness
defaulted trivial AIs back to build_queues.xml
efficiency and slot branching on govs

updated all govs to use same slot logic
all govs retrieve efficiency rating (but dont yet use it)
adding structures.txt to mod
AmmoDepot structure changes

changed relevant tech from Cargo to ProjWeapons
ammo storage starts at 10 secs of production and increases with lvl
construction cost per bullet now static
mass now tied to build costs (and higher)
labor cost no longer static
AnalyzePlanet and Metal Gov changes

AnalyzePlanet updated to compute fuel/ammo production rates
Megal Gov updated:
- to no longer leave any fuel depots
- no longer build cargobay
- to only leave cargobay on 14+ slot planets
- to leave ammo depots as long as metal cost is locally produced