A tiny escript repository on the gist: https://elixirforum.com/t/cant-find-the-module-in-the-escript-made-by-one-mix-exs-file-at-runtime/

cli.ex

defmodule MyApp.CLI do
  def main(_args) do
    IO.puts("Hello from MyApp!")
  end
end

mix.exs

defmodule MyApp.Mixfile do
  use Mix.Project

  def project do
    [app: :my_app,
     version: "0.0.1",
     elixirc_paths: ["."],
     escript: escript()]
  end

  def escript do
    [main_module: MyApp.CLI]
  end
end

If you're on Elixir 1.4+, you can install using:

mix escript.install git https://gist.github.com/d4d44132354a0dae13f1d5b47ef2c2c6.git