Introduce `wp_using_ext_object_cache()` - mimic `wp_suspend_cache_invalidation()` and discourage direct access to `$_wp_using_ext_object_cache`, cleaning up importing of globals in functions and provides function to modify that global.  Loads the packaged object cache when an external cache hasn't been loaded or doesn't contain `wp_cache_init()`.

Fixes #21401.


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