Customize: Defer populating `post_name` for `auto-draft` posts in customized...

Customize: Defer populating `post_name` for `auto-draft` posts in customized state until posts are published.

The ultimate `post_name` is stored in postmeta until the post is published. The `get_page_by_path()` function does not exclude `auto-draft` posts. Revert changes to `wp_unique_post_slug()` from [39411] which excluded `auto-draft` posts.

Props westonruter, dlh for testing, helen for testing.
See #38114, #38928.
Fixes #39078.

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git-svn-id: https://core.svn.wordpress.org/trunk@39446 1a063a9b-81f0-0310-95a4-ce76da25c4cd