Provides a consistent way to query `$wpdb->blogs` for `WP_Site` objects based on domain, path, site ID, network ID, and more.
Introduces and uses `update_site_cache()` and `_prime_site_caches()` to maintain a cached list of `WP_Site` objects for use in multiple queries.
Props spacedmonkey, flixos90, DrewAPicture, jeremyfelt, ocean90.
See #35791.
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