Prohibit compilation if there is a .gleam file and a .erl file with the same file/module name

[inoas]

in src/gleamish_extlib.gleam, I had:

import gleam/io

pub fn main() {
  io.println("Welcome to a Gleamish ExtLib!")
  io.println(
    "Notice: This package is meant to be used only as library and not a stand-alone application.",
  )
}

as well as a gleamish_extlib.erl with -module(gleamish_extlib). and gleam test --target javascript worked but gleam test --target erlang failed and I didn't know why until I transferred one by one files from the current to an empty new project.

[lpil]

Sounds good!

[lpil]

Closing as duplicate of https://github.com/gleam-lang/gleam/issues/1267