Feeling very silly, because I read it months before somewhere and was very easy. Now I can’t find it in docs or forum, all I get is about native extensions.
Was something about declaring some of my folders as extension somehow. Then share the project on github. That’s simple what I want to do, to share some Lua modules between my projects and to the public.
Regards
Look at this project as an example
An important thing to do is include a game.project in the root with the folders you want to include when the project’s zip is added as a dependency.
[library]
include_dirs = defmath
Then in the Lua module the general pattern is
local M = {}
...
return M
For example https://github.com/subsoap/defmath/blob/master/defmath/defmath.lua
Perfect! That’s what I needed. And perfect example.
The right keyword to search is Library instead of Extension, that was my confusion. Was there all the time 
: https://defold.com/manuals/libraries/
Thank you!