2018 path clarity: When path lookup fails because of shadowed crate, provide hint

[alercah]

In Rust 2018, the new path system allows a local name to shadow a crate name:

use std::num;
struct Newtype(num::BigInt); // ERROR

The error is because num refers to the local name of the num module imported on the first line, so num::BigInt fails to look inside the crate. This can be avoided by using extern::num::BigInt, which explicitly refers to the crate (note that @aturon proposed here to make ::num::BigInt be the disambiguation syntax; this doesn't affect the feature request).

The compiler provides no hint of this, however. It could, upon path failure, provide an information along the lines of "num refers to the name declared here; the num crate contains BigInt, use extern::num::BigInt to refer to it" (except written more clearly).

EDIT: The same could apply to any other case where shadowing hides a name that's used as a prefix in a path, but crates seem like the most important.