configured-off modules are required to exist

``` rust
// test.rs
#[cfg(foo)]
#[path = "foo.rs"]
pub mod backend;

#[cfg(not(foo))]
#[path = "bar.rs"]
pub mod backend;
```

If `foo.rs` does not exist, this fails to compile, even if `bar.rs` exists and the `foo` configuration option isn't given. Seems like only modules that are actually being compiled should be required to exist.
