don't display configuration instructions if already configured

[globau]

if .cargo/config is already correctly configured, cargo-vendor shouldn't display instructions stating that it needs to be configured.

[alexcrichton]

👍

This'll probably be done by adding a clause here and likely reloading configuration to not remove source and probing for a few keys.

[alexcrichton]

This crate is now included natively in Cargo as of rust-lang/cargo#6869 and today's nightly Rust. I'm closing all issues in this repository before I archive the repository, but if you're still interested in seeing this issue fixed then please feel free to file an issue with Cargo itself!