Sync Package: Moves the Users and Themes sync module

[roccotripaldi]

We are trying to move away from class mapped packages, and unspool all of the dependencies within a package.

So let's do this!

Part of #12712

Changes proposed in this Pull Request:

This PR should have no functional changes. The Users and Themes sync modules should function exactly the same as before.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

This modifies an existing part of Jetpack.

Testing instructions:

• Try this branch out on a Jurassic.ninja site or on your docker testing site.
• Connect the site and activate the recommended features.
• Point your testing site to your .com sandbox to observe sync actions in real time.
• Try a full sync and ensure there are no php errors in your site's error logs
• Try adding, deleting, update some users on your site. Ensure you see the expected sync actions in the pipeline.

Proposed changelog entry for your changes:

• None

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

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