scouting: if we're trying to go somewhere that is unreachable, reset the destination so we can find a new one
combat: if we don't know of any enemy buildings, always be aggressive. If we don't know of any enemy units, enter a search and destroy mode where we use all available units to scout around the map.
combat: only enter the search and destroy mode if we know of no enemy buildings *and* units
combat: removed a reachability check for adding enemy units to groups.
It was a performance hog since it's the only reason pathing was calculated for enemy units, and it introduced some problems (that could probably be solved by using pathing_map_index::no_enemy_buildings). Either way I am unsure if the check is beneficial at all, but I might experiment with it later, so leaving it commented out for now.
resource_gathering: don't get the pathing distance for a resource (which can't move), use scv instead
combat: workaround for group reachability bug: if we have no non defensive reachable groups, then just mark all groups as reachable