[Rust 2018] pub use can't export crates

[CryZe]

If instead of pub extern crate some_crate; you use pub use some_crate; in Rust 2018, the crate isn't actually reexported.

[aturon]

cc @petrochenkov

[steveklabnik]

Got bit by this today. It's extra bad because it should work... took me a while to convince myself this was a bug, and it wasn't until I asked on Discord that it was confirmed.

[pnkfelix]

as with #52141: visited for triage. P-high. Not sure who is going to work on it; @petrochenkov do you think you have space on your plate to look at this?

[petrochenkov]

The reason is probably that resolution for single-segment imports is implemented as a special case in fn finalize_import, it probably should happen in some other function that's called earlier than "finalize".

If someone can investigate this now, please do.
I'll be able to do it in 3-4 weeks.