In order to assist in the creation of HCP Packer Registry Image metadata
for various Artifacts. A new package is being added to assist in the
  creation of a registryimage.Image from an existing Artifact.

The FromArtifact function provides a simple approach of extracting the
required bits from an Artifact needed by the HCP Packer registry. The
default values, which can be overwritten by the use of a
ArtifactOverrideFunc , were selected to provide some standard defaults
that are set to be available for most plugins were bundled
with Packer prior to v1.7.0. When those defaults are not viable the
package provider override funcs that can be used by the consumer for
setting the appropriate values.

For those  plugins that generate more than one registryimage.Image per
Artifact one can construct a []registryimage.Image and return that
information as the response of Packer core calling
artifact.State(registryimage.ArtifactStateURI).

* Move pkg around to remove a bit of the stutter.
* Add some example tests for a few of the functions provided by the
  package.