In this case, the exact extern crate isn't very important.
This is part of the changes needed to address the spurious failures from
a main test `../removing-extern-crate.rs` being both an auxiliary and a
main test file, causing fs races due to multiple `rustc` processes in
multiple test threads trying to build the main test file both as a main
test and also as an auxiliary at around the same time.
74ba9cbae4