Use RecursiveDataStructureTraverser from wp-cli package

[thelovekesh]

Feature Request

• Yes, I reviewed the contribution guidelines.

For reuse elsewhere, the RecursiveDataStructureTraverser and NonExistentKeyException classes(currently part of this package) have been moved to the wp-cli package in wp-cli/wp-cli#5864.

Once these changes are live, we can safely deprecate or extend the logic from wp-cli package.

See wp-cli/wp-cli#5837

[danielbachhuber]

We'll need to hold off on this until WP-CLI 2.10.0 (or 3.0.0) is released.

[thelovekesh]

Since this has landed in the WP CLI 2.10.0 release and was fixed in #442 for comptaibility, I am closing this as done.