By providing distinct files for the site and network APIs, a better overview is provided. Prior to this change, the `ms-blogs.php` file had grown too big, mixing site APIs, network APIs and related legacy APIs that need to be maintained. Since multisite is often used in unexpected ways, backward-compatibility is ensured by including the two new files from `ms-blogs.php`, which previously contained all functions that have been moved to the new files.
This changeset does not contain any functional changes.
Fixes#40647.
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