Clean up imports; fixup a couple missed errors

[amisevsk]

• Add aliases to oci imports (i.e. use 'ocispec' instead of 'v1')
  • This matches what e.g. oras does internally and makes it easier to track where definitions are coming from (e.g. our store vs. their store)
• Replace functions that create empty structs (same thing as just creating the struct directly)
  • Maybe me being opinionated here, but in Go the struct is public and we don't need new() functions unless there's initialization that needs to happen.
• Catch a missed error in build that hid the fact that running build twice fails

[amisevsk]

Catch a missed error in build that hid the fact that running build twice fails

❯ jmm build -f examples/onnx/Jozufile .
build called
sha256:f23d3cc937341a84aedcd7a0f06654aa656676c3b2222d5435aa361b5c183a62: application/vnd.jozu.model.config.v1+json: already exists

[gorkem]

lgtm. Good call on the New functions, I started those thinking I would need to set some protected items but we cross that bridge when we get there