The block editor needs to duplicate the classic meta box behaviour, so it can extract the registered meta boxes, and import them into the block editor.
To match the classic editor behaviour as closely as possible, this moves the relevant code from the classic editor, into a new function, so it can be called by both.
Merges [43837] from the 5.0 branch to trunk.
Props pento, peterwilsoncc.
Fixes#45172.
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