Use `mod` where possible

Example:

```
use std::fmt;
use std::fmt::Show;
```

becomes

```
use std::fmt::{mod, Show};
```
auto merge of #430 : aochagavia/cargo/mod, r=alexcrichton

Example:

```
use std::fmt;
use std::fmt::Show;
```

becomes

```
use std::fmt::{mod, Show};
```