If we are building from the sdist, we are already in package form.

Fixes #500.

Change-Id: Ie13464dcb4bee249acc833d60b7fb15f65eeaf34
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63430
Reviewed-by: Alejandro Sedeño <asedeno@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>