Move importer types into clientlib importer

[markbiek]

This is the last piece of moving all of the importer definitions into a single area

Fixes #90635

Proposed Changes

In #91609, we refactored all of places that define different importers into a single area. As a final step, this moves importer-related types into the same area.

Why are these changes being made?

Previously, there were multiple lists of importers in different places. Having a single source of truth will make it easier to keep track of things and make updates.

Testing Instructions

Since this is just moving import definitions around, visual inspections and verifying all of the CI actions pass should be sufficient.

However, you could follow the testing instructions in #91609 as an optional, additional check.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

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[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[nelsonec87]

LGTM!