[Feature] Support OCI registries for packages

It would be nice to be able to store packages in and use packages from an OCI registry.

This issue/request is to support the `oci://` URI in addition to `package://` for dependencies.

As for OCI artifacts and layer media types, pkl could adopt vendor based media types for each layer, if each generated artifact was added as a separate layer, such as
`application/vnd.apple.pkl.metadata.v1.xml` for the metadata artifact
`application/vnd.apple.pkl.metadata.v1.checksum_sha256` for the metadata checksum
`application/vnd.apple.pkl.package.v1.zip` for the actual package artifact
and
`application/vnd.apple.pkl.package.v1.checksum_sha256` for the package checksum

This would possibly also require a change to the `pkl-cli` to add a flag to dictate how the package will be build (default | OCI)

Start with pkl-cli and the dependency handling for package:// URIs. Determine the registry and artifact requirements before implementation. Done means dependencies can use oci://, OCI artifacts use the agreed media types, and package builds can select the default or OCI format.

Written by the indexing model from the issue text.