Make unreachable_pub an epoch lint

[Manishearth]

r? @nikomatsakis

[nikomatsakis]

Do we want this to be an epoch lint? This is basically enabled by the new crate keyword, right? It seems like the use of pub vs crate is something we just want to encourage generally.

I admit to feeling somewhat confused about the "lint for epoch" story here.

cc @rust-lang/lang

[Manishearth]

So am I, see #48796

We have two kinds of epoch lints, it's unclear how each should be handled

[Manishearth]

Thinking about this more, neither this nor the dynamic trait lint should be epoch lints; instead, both should be normal lints perhaps under a "rust_2018_idioms" group. Only the keyword lint should be an epoch lint

[Manishearth]

superseded by #48796

(we also changed what we were doing here)