[@rushstack/packlets-eslint-plugin] Add support for ESM-style import paths to packlets eslint plugin

[dmaretskyi]

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ES-modules forces you to reference index.js file when importing a directory:

import { foo } from '../../packlets/foo/index.js';

This PR updates the packlets plugin to treat those as valid imports from the packlet root.

Summary

Details

How it was tested

[ethanresnick]

@octogonz Out of curiosity, what's the status of this? I'd love to use packlets, but this seems like a blocker for me.

[octogonz]

@ethanresnick I think it just slipped through the cracks 😅

Are you able to test it? If it works for you, then we can merge+release it.

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