Auto merge of #50665 - alexcrichton:fix-single-item-path-warnings, r=oli-obk

rustc: Fix `crate` lint for single-item paths

This commit fixes recommending the `crate` prefix when migrating to 2018 for
paths that look like `use foo;` or `use {bar, baz}`

Closes #50660

diff --cc src/librustc_lint/lib.rs
Simple merge

diff --cc src/librustc_resolve/lib.rs
Simple merge