Remove crate_id support from `extern crate`

It looks like crate ID's are unlikely to carry over into the cargo world. Right now our `extern crate` syntax supports a form like

```
extern crate foo = "foo:1.0#path"; // or something similar
```

Remove the `= "string"` syntax.

Because we do still need to support renaming, add something like the following:

```
extern crate foo = bar;
```

Nominating.
