x/tools/gopls: can't jump-to-definition on embedded type alias

Setup:
```
type T = struct{}

type S struct {
  T
}

var x T
```

Jump to definition works on the `T` from `var x T`, but not on the `T` embedded in `S`. I'm sure that this is due to the way go/types handles type aliases, but we should work around this in the syntax layer.
