Fix crash for certain re-exports in `cargo-check-external-types` (#1654)

The visitor was examining the inner contents of the entire re-exported
type, which, for a type that is re-exported within the same crate, is
the correct behavior. Its the wrong behavior when exporting types from
other crates, however, since it doesn't matter what is inside that
external type, and rustdoc doesn't include information about inner
pieces of the external type in the JSON output.