Include erlang/elixir ffi modules in exported erlang app files

[LostKobrakai]

This does seem to make the compiler happy, but I'm kinda lost on where and how to test this kind of behaviour.

Related to #3710

[lpil]

Hey! Thanks for looking into this. 💜

It's a bit tricky to test this bit as it involves code generation, but you could add some checks to this test script: https://github.com/gleam-lang/gleam/blob/main/test/external_only_erlang/test.sh

I think your change here may have stopped the entrypoint module from getting compiled by the way- the Erlang compilation is being run after it is rendered to disc. Perhaps the .app writing could be moved to be the last thing in the module rather than moving the Erlang compilation up?

[LostKobrakai]

Somehow this broke the project_erlang test, but I compared the before and after of the resulting folders and they seems to be the same (besides obvious differences in cache files and the compiler script)

[lpil]

Super nice work!!! Really digging this.

I've left some small notes inline