use real function calls instead of the old string method where possible

(basically leaving only the old legacy mapgen calls still using the string method)

youngtrees/init.lua

@@ -150,5 +150,5 @@ plantslib:register_generate_plant({
     min_elevation = 1, -- above sea level
 	plantlife_limit = -0.9,
   },
-  "abstract_youngtrees.grow_youngtree"
+  abstract_youngtrees.grow_youngtree
 )