No longer able to generate bindings in submodule

The following works in the latest release but not on main:

```wit
package my-ns:my-package;

interface my-interface {
my-func: func();
}

world my-world {
import my-interface;
export my-interface;
}
```

```rust
mod bindings {
wit_bindgen::generate!({
path: "wit",
});
}

use bindings::exports::my_ns::my_package::my_interface::Guest;

bindings::export!(Impl);

struct Impl;

impl Guest for Impl {
fn my_func() {}
}
```

On the main, the following error shows:

```
error[E0433]: failed to resolve: could not find `export` in the crate root
--> src/lib.rs:3:5
|
3 | / wit_bindgen::generate!({
4 | | path: "wit",
5 | | });
| |______^ could not find `export` in the crate root
...
10 | bindings::export!(Impl);
| ----------------------- in this macro invocation
```

Seems we're now assuming something lives at the crate root which we did not assume before.