Reorganize docker/internal/tarfile.Writer.createManifest a bit
Reorganize docker/internal/tarfile.Writer.createManifest a bit

... only to make future commits easier to review.

This introduces a bit of inefficiency (creating an on-stack
ManifestItem only to copy it into a single-element array), but
that will be gone momentarily.

Should not change behavior.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image