Imports of package purl

PathSynopsis
github.com/containerd/containerd/platforms
github.com/docker/distribution/referencePackage reference is deprecated, and has moved to github.com/distribution/reference.
github.com/opencontainers/go-digestPackage digest provides a generalized type to opaquely represent message digests and their operations within the registry.
github.com/opencontainers/image-spec/specs-go/v1
github.com/package-url/packageurl-goPackage packageurl implements the package-url spec
github.com/pkg/errorsPackage errors provides simple error handling primitives.
stringsPackage strings implements simple functions to manipulate UTF-8 encoded strings.