Fix addons not working when resolved with getAbsolutePath

[JReinhold]

Follow-up to #63

In certain monorepo and/or Yarn PnP situations, users will resolve their addon packages with an absolute path instead of the package name, eg.:

// .storybook/main.js
import { join, dirname } from "path";

function getAbsolutePath(value) {
  return dirname(require.resolve(join(value, "package.json")));
}

const config = {
  ...,
  addons: [
    getAbsolutePath("storybook-addon-kit-test-module"),
  ],
  ...
}

In that situation, Node.js will ignore the package's export maps when resolving it and thus none of the entries will be found. To get around that we have to have preset.js, manager.js and/or preview.js files in the root package because they will be used instead.

This has always been the case, we just missed this scenario in #63 and removed the files by mistake.