Support phantom types inside a method body

The poor interaction between method bodies and handling of relative paths in `use` statements in Rust makes this extremely challenging or (I currently believe) impossible to support. Doesn't hurt to have an issue though:

```rust
use ghost::phantom;

fn main() {
#[phantom]
struct MyPhantom;
}
```

```console
error[E0432]: unresolved import `super::MyPhantom`
--> src/main.rs:5:12
|
5 | struct MyPhantom;
| ^^^^^^^^^ maybe a missing `extern crate MyPhantom;`?

error[E0432]: unresolved import `self`
--> src/main.rs:4:5
|
4 | #[phantom]
| ^^^^^^^^^^ maybe a missing `extern crate __value_MyPhantom;`?
```

Start by reproducing the Rust example in the issue and examine how the method body interacts with relative paths in `use` statements. The issue does not name implementation files or tests; done would mean supporting the shown `#[phantom]` declaration inside `main` without the unresolved-import errors.

Written by the indexing model from the issue text.