`extern crate ... as std` overrides the injected crate on 2018 edition

[matthewjasper]

On the 2018 edition the following code:

extern crate some_crate as std; // or as core with #![no_std]

will:

• Try to resolve prelude::v1::* in some_crate, failing if it doesn't exist. This probably limits the plausible cases where this could happen to extern crate core as std, and extern crate core as std with #![no_std].
• Use some_crate in any built-in derives and for HIR lowering.

I would expect that the prelude and derives/HIR lowering to always use the injected std/core. Paths starting with std should still resolve to the in the crate the user provided.

In the 2015 edition it creates a name conflict with the implicit extern crate std.

cc @petrochenkov @eddyb

[Centril]

This would be fixed by #60607.