Bundle nested FFI on publish

[PgBiel]

Fixes #4238

Not sure if erlang_files should be modified as well, let me know if it'd be better to remove it from this PR.

[lpil]

Are you ready for a review? :)

[PgBiel]

Not yet, I'll be experimenting a bit further with include/ still, that's the only missing part. I'll let you know once it's ready!

[PgBiel]

Okay, I figured it out: the exported include/ directory is auto-generated on erlang codegen and is always flat (at least, the files produced by Gleam always appear at the top-level - not sure if erl can produce anything there, but I couldn't get it to at first), so 1. no need to export nested files and 2. no need for .gitignore checks (since the auto-generated dir is inside build/, so it would always be ignored...).