cfg diagnostic not applied correctly

I tried this code:
`cargo add rsa`
```rust
fn main() {
    println!("Hello, world!");
    rsa::sha2::Sha256;
}
```
```
error[E0433]: failed to resolve: could not find `sha2` in `rsa`
 --> src/main.rs:5:10
  |
5 |     rsa::sha2::Sha256;
  |          ^^^^ could not find `sha2` in `rsa`
```

the cfg diagnostic is missing, although the code path for `find_cfg_stripped` seems to be hit.