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;
  }
}

[chinedufn]

Looks good thanks.

Please just update the PR body to show an example of a bridge module that:

1. was not possible before this PR
2. is now possible as of this PR

Example: #211

---

We use these examples:

• when writing release notes
• when linking new contributors to old PRs to use as inspiration
• for anyone that watches the repo to be able to get a quick sense of new changes without needing to read code

---

Once the PR body is updated we can land this.

Can just do

This commit ... what it does ...

For example, the following is now possible:

EXAMPLE BRIDGE MODULE HERE

[jnbooth]

@chinedufn Done. Thank you for providing the example!

[chinedufn]

Ooo nice example.

Thanks for submitting and landing your first contribution!