Allow generating public modules for Rust on import

[philpax]

Right now, if you generate Rust-import, you get this:

#[allow(clippy::all)]
mod input {

If you generate wasmtime-import, you get this:

#[allow(clippy::all)]
pub mod input {

Ideally, there'd be an option to make the former generate like the latter (e.g. with pub mod instead of mod), or they would both be pub mod by default.

With the modules being private, I can't expose them to user code using their original names. I'd use wit_bindgen_wasmtime::import to sidestep this, but I encounter build issues:

error[E0425]: cannot find function `lock` in module `os`
  --> .cargo/registry/src/github.com-1ecc6299db9ec823/region-2.2.0/src/lock.rs:30:7
   |
30 |   os::lock(
   |       ^^^^ not found in `os`
   |
help: consider importing this function
   |
1  | use lock;
   |
help: if you import `lock`, refer to it directly
   |
30 -   os::lock(
30 +   lock(
   |

error[E0425]: cannot find function `unlock` in module `os`
  --> .cargo/registry/src/github.com-1ecc6299db9ec823/region-2.2.0/src/lock.rs:60:7
   |
60 |   os::unlock(
   |       ^^^^^^ not found in `os`
   |
help: consider importing this function
   |
1  | use unlock;
   |
help: if you import `unlock`, refer to it directly
   |
60 -   os::unlock(
60 +   unlock(
   |

error[E0425]: cannot find function `page_size` in module `os`
  --> .cargo/registry/src/github.com-1ecc6299db9ec823/region-2.2.0/src/page.rs:16:39
   |
16 |     INIT.call_once(|| PAGE_SIZE = os::page_size());
   |                                       ^^^^^^^^^ not found in `os`

error[E0425]: cannot find function `set_protection` in module `os`
  --> .cargo/registry/src/github.com-1ecc6299db9ec823/region-2.2.0/src/protect.rs:48:7
   |
48 |   os::set_protection(
   |       ^^^^^^^^^^^^^^ not found in `os`

error[E0425]: cannot find function `get_region` in module `os`
   --> .cargo/registry/src/github.com-1ecc6299db9ec823/region-2.2.0/src/lib.rs:133:7
    |
133 |   os::get_region(page::floor(address as usize) as *const u8)
    |       ^^^^^^^^^^ not found in `os`

(I also don't know which one I'm supposed to use for a Wasmtime export and a Rust guest, anyway)

[Liamolucko]

Perhaps import could be changed to an attribute macro on mod, so that you can explicitly add the pub modifier, e.g.:

#[wit_bindgen_rust::import("foo.wit")]
pub mod foo;

[Michael-F-Bryan]

It's not a solution, but one trick I've been using is to re-export your generated module's contents under a module with the same name.

pub mod foo {
  wit_bindgen_wasmtime::import!("foo");

  pub use self::foo::*; 
}