fix: Adds multis to map iterators, fixes searching empty nested containers (#1581)

### Summary
Eliminates `IPooledEnumerable<BaseMulti>` and `eable.Free()` from `Map` for multis. This drastically simplifies code that iterates in range, for example:

```cs
foreach (var m in m.GetMultisInRange(5))
{
}
```
The code above no longer requires an eable and calling `Free()`.


### Bug Fixes
- [X] Fixes an issue with searching through nested empty containers.

Projects/UOContent/Spells/Base/SpellHelper.cs

@@ -158,12 +158,8 @@ namespace Server.Spells
                 return false;
             }
 
-            var sector = map.GetSector(p.X, p.Y);
-
-            for (var i = 0; i < sector.Multis.Count; ++i)
+            foreach (var multi in map.GetMultisAt(p))
             {
-                var multi = sector.Multis[i];
-
                 if (multi is BaseHouse bh)
                 {
                     if (houses && bh.IsInside(p, 16) || housingrange > 0 && bh.InRange(p, housingrange))