Apply input module visibility to output module

[jnbooth]

Addresses #252.

Here's an example of using this.

// enums.rs

#[swift_bridge::bridge]
pub mod ffi {
    enum MyEnum {
        VariantA,
        VariantB,
        VariantC,
    }
}

// lib.rs
mod enums;
use enums::ffi::MyEnum;

#[swift_bridge::bridge]
mod ffi {
  #[swift_bridge(already_declared)]
  enum MyEnum {}

  extern "Rust" {
    fn getEnum() -> MyEnum;
  }
}

[jnbooth]

@chinedufn Done. Thank you for providing the example!

[chinedufn]

Ooo nice example.

Thanks for submitting and landing your first contribution!