Use a more defensive vendored archive/tar import path to make it clear this isn't upstream's released archive/tar

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4369 from tianon/better-archive-tar-vendor-import-path

Use a more defensive vendored archive/tar import path to make it clear this isn't upstream's released archive/tar