Proposal: `?manifest` query to get JSON containing full list of files

AFAIK, currently there's no way to get what files are present in a specific release of a library. If one really want this, they has to create and include a dedicated file for it to the release, *manually* maintaining a full list of files like:

```json
{
"files": [
"mod.ts",
"README.md",
"LICENSE",
"assets/image.png"
]
}
```

This “build step” can be automated of course, but people would quite oftenly mistake or forget to do that. It would be good if **deno.land generates this file on the fly and caches it permanently for each version**, by using the query parameter `?manifest`, possibly with a version specifier of manifest specification, e.g. `https://deno.land/std@0.191.0?manifest=v1`.

Tell me if there's already a feature or a future-proof workaround to get a similar result.