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

Erroneous code example:

```ignore (Multiple source files are required for compile_fail.)
// file: ambiguous_module/mod.rs

fn foo() {}

// file: ambiguous_module.rs

fn foo() {}

// file: lib.rs

mod ambiguous_module; // error: file for module `ambiguous_module`
                      // found at both ambiguous_module.rs and
                      // ambiguous_module.rs/mod.rs
```

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