Document all the places where packages are listed or filtered

[gziolo]

This issue was discussed several times in the past few weeks. It was raised again by @aduth yesterday in this comment:

I'm wondering if we ought to start codifying / documenting where we have these conventions, i.e. "sources should have a src/index.js file".

There are a few places where we execute some logic based on the list of packages which is either hardcoded or manually crafted, some examples:

• Improve handling of transpiled packages in unit tests #14432 (comment) (webpack.config.js)
• https://github.com/WordPress/gutenberg/blob/master/webpack.config.js#L77-L84 (webpack.config.js)
• https://github.com/WordPress/gutenberg/blob/master/test/unit/jest.config.js#L7-L8 (jest.config.js)
• https://github.com/WordPress/gutenberg/blob/master/docs/tool/manifest.js#L9-L12 (docs tool manifest)

We should at least document all those places to ensure we update all of them when necessary. Ideally, we abstract away as much as possible and start using helper methods which makes maintenance easier.

[gziolo]

It's automated as of today - those list are generated with pattern matching.