Compiles the project and writes <app>_<vsn>.pplt at the root of each of its applications, made from the beams that were just compiled (with the hash of the sources, so that whoever uses it can tell if it is stale). It also leaves in _build/ the .pplt of the dependencies that nobody shipped.

Add the file to what you upload to Hex (files: in mix.exs), and run this task before mix hex.publish.