Reading function names from other files is something the language server/extension for VS Code has to do, and I have yet to find one for Lua that does it.
When v3 is ready to go I'd definitely like to add a set of autocomplete snippets to the existing pack here:
https://forum.cockos.com/showthread.php?t=208769 , but until then you're out of luck. Keeping modules small does help with remember what functions they expose though.