LSP: Support triggering find references from imports

In the following example, triggering find references on any of the underlined locations should show all of the uses of the type Foo. It currently works everywhere but in the import line.

// -- foo.gleam
pub type Foo
//       ^^^

// -- app.gleam
import foo.{type Foo}
//               ^^^ ← this line shows up in all results, but triggering here fails
fn main() {
  let _: Foo = todo
  //     ^^^
}