[Proposal] Improve the catalog endpoint format

Is your feature request related to a problem? Please describe.

Right now, utilities that want to use the /catalog endpoint often need to aggregate each module's versions into a list. For example, the /catalog endpoint returns this:

{
    "modules": [
        {
            "module":"github.com/arschles/assert",
            "version": "v1.0.0"
        },
        {
            "module": "github.com/arschles/assert",
            "version": "v1.1.0"
        }
    ]
}

Many programs will find it more convenient to get a response similar to this:

{
    "modules": {
        "github.com/arschles/assert": ["v1.0.0", "v1.1.0"]
    }
}

Describe the solution you'd like

I'd like us to add a V2 of the catalog API that responds this way. This could possibly be exposed as a /catalog/v2 endpoint.

Describe alternatives you've considered

Clients can always aggregate in their own code

Additional context

• There will be non-trivial work on the storage systems to achieve this
• There are two projects I know of that could benefit from this change:
  • moddoc
  • crathens

cc/ @marwan-at-work