Units can end up inside the foundation they're building

[wraitii]

Related to #5084 but not quite the same.

The "units on top of the foundation" check is non-strict to avoid detecting builders since ​Phab:rP17161 / ​Phab:rP17225 (eg A21?). To be honest, I'm not entirely sure if that's actually needed, and might need reverting.

I don't think one can use this to actually go OOB, since the short-range pathfinder is used and that checks for obstructions. _Maybe_ by placing units fancily one could switch from an obstruction to another, but crossing into invalid terrain is likely impossible (couldn't do it anyways).

it's somewhat easily reproducible by using formations, but they aren't actually related to the problem