lua packages: add init.lua to modules search path

pkgs/development/lua-modules/generic/default.nix

···
           if [[ $LUA_PATH = *"$package_path"* ]]; then return; fi
 
           if [[ -z $LUA_PATH ]]; then
-            export LUA_PATH="$package_path/?.lua"
+            export LUA_PATH="$package_path/?.lua;$package_path/?/init.lua"
           else
-            export LUA_PATH="$LUA_PATH;$package_path/?.lua"
+            export LUA_PATH="$LUA_PATH;$package_path/?.lua;$package_path/?/init.lua"
           fi
         }