Scripts: Ensure sub-directory WordPress installs can load polyfill scripts.

`wp_get_script_polyfill()` bypasses `WP_Scripts::do_item()`, so didn't transform the script path into its fully qualified URL.

Props swissspidy, ocean90.
Merges [43960] to trunk.
Fixes #45469.
Built from https://develop.svn.wordpress.org/trunk@44286


git-svn-id: https://core.svn.wordpress.org/trunk@44116 1a063a9b-81f0-0310-95a4-ce76da25c4cd