Revert the revert of the Move of `RecursiveDataStructureTraverser` to `wp-cli/wp-cli` by schlessera · Pull Request #5871

Revert the revert of the Move of `RecursiveDataStructureTraverser` to `wp-cli/wp-cli` by schlessera · Pull Request #5871 · wp-cli/wp-cli

This reverts commit a4303bc (which reverted commit d0d2a34 😅)

See discussion in #5866

It also changes the namespaces to be singular, as that is the best practice in PHP. I'ma ware that we already have plural namespaces in the package, but that doesn't preclude us from doing it correctly for all new subnamespaces, given that a consumer will not see all other subnamespaces anyway.