Introduce builtin_shared and use it to fix #4778

Fixes #4778 which was about the error:

ServerError: Lua: Runtime error from mod '' in callback item_OnPlace():
/usr/local/share/minetest/builtin/game/item.lua:278: attempt to call global 'check_attached_node' (a nil value)

The issue was a regression of commit 649448a2a9

"Rename nodeupdate and nodeupdate_single and make them part of the official API"

builtin/game/init.lua

@@ -3,10 +3,14 @@ local scriptpath = core.get_builtin_path()..DIR_DELIM
 local commonpath = scriptpath.."common"..DIR_DELIM
 local gamepath = scriptpath.."game"..DIR_DELIM
 
+-- Shared between builtin files, but
+-- not exposed to outer context
+local builtin_shared = {}
+
 dofile(commonpath.."vector.lua")
 
 dofile(gamepath.."constants.lua")
-dofile(gamepath.."item.lua")
+assert(loadfile(gamepath.."item.lua"))(builtin_shared)
 dofile(gamepath.."register.lua")
 
 if core.setting_getbool("profiler.load") then
@@ -21,7 +25,7 @@ dofile(gamepath.."auth.lua")
 dofile(gamepath.."chatcommands.lua")
 dofile(gamepath.."static_spawn.lua")
 dofile(gamepath.."detached_inventory.lua")
-dofile(gamepath.."falling.lua")
+assert(loadfile(gamepath.."falling.lua"))(builtin_shared)
 dofile(gamepath.."features.lua")
 dofile(gamepath.."voxelarea.lua")
 dofile(gamepath.."forceloading.lua")