Assert that both parts of mcl_autogroup exist

mods/CORE/_mcl_autogroup/init.lua

@@ -69,6 +69,8 @@ This also means that it is very important that no mod adds _mcl_autogroup as a
 dependency.
 --]]
 
+assert(minetest.get_modpath("mcl_autogroup"), "This mod requires the mod mcl_autogroup to function")
+
 -- Returns a table containing the unique "_mcl_hardness" for nodes belonging to
 -- each diggroup.
 local function get_hardness_values_for_groups()

mods/CORE/mcl_autogroup/init.lua

@@ -12,6 +12,8 @@ as possible.  Minetest loads mods in reverse alphabetical order.
 mcl_autogroup = {}
 mcl_autogroup.registered_diggroups = {}
 
+assert(minetest.get_modpath("_mcl_autogroup"), "This mod requires the mod _mcl_autogroup to function")
+
 -- Register a group as a digging group.
 --
 -- Parameters: