hof/mod: support module mirrors

[verdverm]

{ oci, git } X { public, private}

we want to test mirroring oci->oci & git->oci too

[verdverm]

I'm not sure we implemented mirrors correctly, how the path or url is represented, I think CUE will do it differently and we should align with that, or we can have a say in how it is done?

I'm not concerned about implementing mirrors yet, we can put that off to the next release, building blocks are here now

[verdverm]

Need to think through the config, also want to account for format, auth, and other extras

This file is part of a users' (or CI) environment, not on a project basis, not part of a module. In other words, your configuration here will not influence downstream users, who will get the same (or different) modules from the same dependency list. We probably need a canonical source of truth for modules, probably in the module.cue

something like

registries: [string]: {
  url: string
  port: int
  format: "git" | "oci"  // git mirror supports forks without the need for a replace
  
  auth: {
    ... stuff to reference or configure method, not the credentials themselves
    we may not need anything here, as we leverage the external tools auth
  }

  modules: [prefix]: {
    // not sure we need anything here, but we'll leave it open, we want a struct anyway, not a list, for easy lookup
  }
}

alternatively, we could reorg this as

module: [prefix=string]: {
  registry config...
}

This might also become a larger hof/config.cue file that holds more stuff, makes sense to have just a single config file