Rollup merge of #50667 - alexcrichton:no-remove-extern-crate-unless-works, r=Manishearth

rustc: Only suggest deleting `extern crate` if it works

This commit updates one of the edition lints to only suggest deleting `extern
crate` if it actually works. Otherwise this can yield some confusing behavior
with rustfix specifically where if you accidentally deny the `rust_2018_idioms`
lint in the 2015 edition it's suggesting features that don't work!

Author: kennytm