image/manifest: Split unpackLayerEntry into its own function
image/manifest: Split unpackLayerEntry into its own function

To help address:

  $ make lint
  checking lint
  image/manifest.go:140::warning: cyclomatic complexity 39 of function unpackLayer() is high (> 35) (gocyclo)
  ...

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/image-tools,opencontainers/image-tools