Auto merge of #3386 - matklad:warn-replace, r=alexcrichton

Warn if replace is not actually used

closes #3324

I've added warnings to `resolve_dependencies` inside `cargo_compile.rs`. Adding them to `resolve_with_previous` does not work because that method is called several times.

Perhaps the best idea is to refactor resolve facade, such that we have only `ops::resolve`  and not `ops::resolve_ws`, `ops::resolve_with_previous` and `ops::resolve_dependencies`. IIRC, there were some bugs because of logic mismatch between `compile` and `metadata` commands.