move default mod to opt-depends and use screwdriver only if available

mod.conf

@@ -1,7 +1,7 @@
 name = scifi_nodes
 description = Minetest mod that adds scifi themed blocks, doors, materials, plants and other assets.
-depends = default
 optional_depends = """
+default,
 xpanes,
 mesecons,
 moreblocks,

nodeboxes.lua

@@ -1080,7 +1080,7 @@ minetest.register_node("scifi_nodes:itemholder", {
 		}
 	},
 	groups = {cracky=1},
-	on_rotate = screwdriver.disallow,
+	on_rotate = minetest.get_modpath("screwdriver") and screwdriver.disallow,
 	after_place_node = function(pos, placer, itemstack)
 		local meta = minetest.get_meta(pos)
 		meta:set_string("owner",placer:get_player_name())