Multiple candidate files were found for an out-of-line module.

Erroneous code example:

```rust
// file: ambiguous_module/mod.rs

fn foo() {}
```

```rust
// file: ambiguous_module.rs

fn foo() {}
```

```ignore (multiple source files required for compile_fail)
mod ambiguous_module; // error: file for module `ambiguous_module`
                      // found at both ambiguous_module.rs and
                      // ambiguous_module.rs/mod.rs

fn main() {}
```

Please remove this ambiguity by deleting/renaming one of the candidate files.
