Populate `entryPoints` from `deno.json#exports` or `jsr.json#exports`

Exports are already declared in `deno.json` and/or `jsr.json`, but are required in `BuildOptions`.
It would be nice if `build()` generated the entry points list.

This is what I have in my build script to do the same:
```js
import { build } from "@deno/dnt"
import denojson from "../deno.json" with { type: "json" }

await build({
//...
entryPoints: Object.entries(denojson.exports).map(([name, path]) => ({
name,
path,
})),
//...
})
```