Exuberant Ctags support for lua does not handle modules properly.
(see e.g. https://awesome.naquadah.org/bugs/index.php?do=details&task_id=1313).
I wonder it would be fairly easy to provide first class support for "goto declaration/definition", e.g. through YouCompleteMe, which has an API for this: https://github.com/Valloric/YouCompleteMe#ycmcompleter-subcommands
Here is the Python implementation, using Jedi: https://github.com/Valloric/ycmd/blob/master/ycmd/completers/python/jedi_completer.py