Sync: PSR-4 the Attachments sync module

[tyxla]

This PR allows for sync modules to be loadable via PSR-4, and updates the Attachments sync module to be loaded via PSR-4.

Changes proposed in this Pull Request:

• Make the sync package modules directory compatible with PSR-4
• Update the Attachments module

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

• Part of the Jetpack DNA project - p1HpG7-70O-p2

Testing instructions:

• Checkout the branch.
• Enable WP_DEBUG_LOG.
• Trigger a full sync.
• Verify sync works well and you have no errors logged.
• Verify tests pass and CI is green.

Proposed changelog entry for your changes:

• Sync: PSR-4 the Attachments sync module

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: July 2, 2019.
Scheduled code freeze: June 25, 2019

Generated by 🚫 dangerJS against 1a9267e

[lezama]

easy peasy lemon squeezy

[lezama]

waiting for the tests to pass before changing labels

[tyxla]

and they passed ✅