Rather than calling out to tar we use the golang tar parser
to directly extract the tar files. This has two major advantages:
1) We're able to replace an existing directory with a file in the
new layer. This currently breaks with the external tar, since
it refuses to recursively remove the destination directory in
this case, and there are no options to make it do that.
2) We avoid extracting the whiteout files just to later remove them.
Ben Wiklund