This makes it so the diff package is depended on by all implementations, rather
than the diff package having to depend on the default myers implementation.
Change-Id: I04b9caee6ff1017fa8e5476a7434e4b0e17753c3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/198379
Reviewed-by: Rebecca Stambler <rstambler@golang.org>