edition lint: migrating `extern crate` with `#[macro_use]`

[nrc]

The migration issue is that #[macro_use] extern crate foo; beings macros into scope from foo and extern crate is unidiomatic in the 2018 edition. local_inner_macros is the current solution but as discussed in #50911, we're not 100% that works.

[nikomatsakis]

I can't tell what the status is here. We have a lint (landed in #52275) but as far as I can tell (playground) it does not fire. In fact, I get no warnings at all when using extern crate in Rust 2018, even with idiom lints enabled. Is that expected?

(We do appear to have stabilized use_extern_macros in #50911.)

[nikomatsakis]

I don't think however that this needs to be a Rust 2018 RC2 blocker, since edition lints are not idiom enabled by default.