Revert "Update `WP_CLI\Utils\load_command()` to handle commands like `sidebar widget`"

This reverts commit 16892f074fc2d688f267299cdfabd63c68e3bc27.

See https://github.com/wp-cli/wp-cli/commit/16892f074fc2d688f267299cdfabd63c68e3bc27#commitcomment-5951766
Rename and refactor to accommodate 77222f7967b989e174b75b2852d9fad66ae90bfa