Test if a lua file is runnnig standalone or is it required

caller_test.lua

-- Obviously, if a standalone file is called with an argument that corresponds to a loaded file, it will think it's required.
-- For example: 
--     lua caller_test.lua _G 
-- will choose the else branch.
-- For our purposes, though, this is good enough.
if not _G.package.loaded[...] then
  print("Running standalone")
else
  print("Required")
end