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This commit is a step towards a goal to make names less ceremonial
outside of the registry client. Clients of the registry package can
treat names as opaque strings, and the registry package will handle
parsing, validating, and normalizing names.

Ideally we end up with the names package tucked away in an internal
package for good. We'll see how things go.

Also, this package name is not permanent. This another step in the
on-going process of refactoring the server code, and at some point it
will most likely be renamed/moved.

This commit introduces a new API implementation for handling
interactions with the registry and the local model cache. The new API is
located in server/internal/registry. The package name is "registry" and
should be considered temporary; it is hidden and not bleeding outside of
the server package. As the commits roll in, we'll start consuming more
of the API and then let reverse osmosis take effect, at which point it
will surface closer to the root level packages as much as needed.