[cargo_new] Hint to use `cargo init` on existing dir

[cargo_new] Hint to use `cargo init` on existing dir

The `new` command always expects a non-existing path. The `init` command
always expects an existing path. Therefore, it makes sense to hint to
use `init` if the pre-condition to `new` is not satisfied.

Fixes <https://github.com/rust-lang/cargo/issues/4366>