Add support to generate package.json during build?

Lerna [recently added](https://github.com/lerna/lerna/tree/master/commands/publish#--contents) support for the `--contents` argument.

Basically, it makes it possible to tell Lerna to publish to npm just the content of the `dist` folder (used as root directory rather than subfolder).

This means that if you have:

```
dist/
index.es.js
index.umd.js
src/
index.es.js
index.umd.js
package.json
```

it will push to npm just the content of `dist` and you'll be able to import stuff form the package with `import umd from 'pkgname/index.umd.js'`

The only missing piece is a way to generate a compatible package.json file that must be put inside the `dist` folder during the build process.

That means that the `main` (and other) field should be changed from `dist/index.js` to `index.js`.

Do you think this is something that Rollup could handle? If so I'd like to work on a PR to add it.