Separating out from #4225 (comment)

In #2609 we added an extension rule for the core no-duplicate-imports. In hindsight - this wasn't a great idea because there already exists an even better rule in the ecosystem - import/no-duplicates.

Instead of us attempting to maintain a competing rule we should instead deprecated our version of the rule and point users at the eslint-plugin-import rule instead.

Similar to camelcase - we can maintain a markdown doc which points users to the better rule.