Deref implementations don't appear if it's implemented on a type alias

For example:

```rust
pub struct Tags(TagsImpl);
/// FlatMap representing the key-value pairs of the tags

pub type TagsImpl = ::flat_map::FlatMap<String, String>;

impl Deref for Tags {
    type Target = TagsImpl;

    fn deref(&self) -> &Self::Target {
        &self.0
        &self.0
    }
}
```

In here, the `Deref` implementation won't appear.