Fix replaceInstances messing up GetChildren ordering (#1265)

Fixes #1257. replaceInstances now preserves GetChildren ordering.

I also had the unit tests unreliably say that Packages wasn't a member
of ReplicatedStorage so I added WaitForChild and tests run 100% for me
now.

plugin/src/orderSwaps.lua

@@ -0,0 +1,44 @@
+--[[
+	Determines the order in which `ServeSession:__replaceInstances` should swap
+	instances so that sibling order is preserved.
+
+	Roblox appends to `GetChildren()` on every reparent, so the order in which we
+	re-parent replacements determines their final sibling order. To rebuild
+	`GetChildren()` exactly as it was before the swap we must:
+
+	* process ancestors before descendants, so each replacement's parent already
+	  exists when we re-parent the replacement, and
+	* process siblings in their original `GetChildren()` order.
+
+	`swaps` is an array of `{ id, replacement, oldInstance }` entries. This sorts
+	the array in place (annotating each entry with `depth`/`siblingIndex`) and
+	returns it.
+]]
+local function orderSwaps(swaps)
+	for _, swap in swaps do
+		local depth = 0
+		local ancestor = swap.oldInstance.Parent
+		while ancestor ~= nil do
+			depth += 1
+			ancestor = ancestor.Parent
+		end
+		swap.depth = depth
+
+		local siblingIndex = 0
+		if swap.oldInstance.Parent ~= nil then
+			siblingIndex = table.find(swap.oldInstance.Parent:GetChildren(), swap.oldInstance) or 0
+		end
+		swap.siblingIndex = siblingIndex
+	end
+
+	table.sort(swaps, function(a, b)
+		if a.depth ~= b.depth then
+			return a.depth < b.depth
+		end
+		return a.siblingIndex < b.siblingIndex
+	end)
+
+	return swaps
+end
+
+return orderSwaps