Sync Package: remove dependency on JETPACK__PLUGIN_DIR from packages

See:
https://github.com/Automattic/jetpack/blob/14a647f75f1acaabac06af78ba0fb46a7fe36262/packages/sync/legacy/class.jetpack-sync-module-options.php#L156

Packaged classes should not require nor depend on code from jetpack.

This issue applies to:

- Automattic\Jetpack\JITM
- Automattic\Jetpack\Sync\Defaults
- Automattic\Jetpack\Sync\Functions
- Automattic\Jetpack\Sync\Listener
- Automattic\Jetpack\Sync\Modules\Options

Start with packages/sync/legacy/class.jetpack-sync-module-options.php at the referenced line, then trace JETPACK__PLUGIN_DIR usage in Automattic\Jetpack\JITM, Sync\Defaults, Sync\Functions, Sync\Listener, and Sync\Modules\Options. Done means these packaged classes no longer require or depend on code from Jetpack.

Written by the indexing model from the issue text.