The `archive` package defines aliases for `io.ReadCloser` and
`io.Reader`. These don't seem to provide an benefit other than type
decoration. Per this change, several unnecessary type cases were
removed.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
Now that the archive package does not depend on any docker-specific
packages, only those in pkg and vendor, it can be safely moved into pkg.
Signed-off-by: Rafe Colton <rafael.colton@gmail.com>
