Do not suggest use over extern crate w/ alias.

Do not suggest use over extern crate w/ alias.

This commit stops `unused_extern_crates` lints from occuring on `extern
crate` statements that alias the crate as the suggestion to change to a
`use` statement would result in the aliased name no longer being added
to the prelude, thereby causing compilation errors if other imports
expected this to be the case.