Auto merge of #5572 - kngwyu:for-racer, r=alexcrichton

Make add_overrides and get_resolved_packages pub

For https://github.com/racer-rust/racer/pull/855.
Now  we're planning to use cargo to resolve dependencies, but for RLS we can't write `Cargo.lock`.
So we have to call `resolve_with_previous` directly, and want to use these functions.