Make mesecons_switch a proper optional dependency (H#175)

advtrains/mod.conf

@@ -4,4 +4,4 @@ description=Core system for realistic trains in Minetest
 author=orwell96
 
 depends=serialize_lib
-optional_depends=mesecons,digtron
+optional_depends=mesecons,mesecons_switch,digtron

advtrains/p_mesecon_iface.lua

@@ -1,7 +1,7 @@
 -- p_mesecon_iface.lua
 -- Mesecons interface by overriding the switch
 
-if not mesecon then return end
+if minetest.get_modpath("mesecons_switch") == nil then return end
 
 minetest.override_item("mesecons_switch:mesecon_switch_off", {
 	groups = {