Remove and replace php-symfony-polyfill-* dependencies

As discussed in #911832, the polyfilled dependencies can safely be replaced with PHP extensions. For non-builtin extensions (apcu, intl, mbstring), we need to manually add the dependency for now to avoid inheriting the version constraint against the polyfill.

All of the packages in the Build-Depends already have their native extensions being installed. The hardcoded autoloader entries in d/*.autoload.php.tpl were also removed.

Finally, this also takes care of the circular dependency between php-symfony-intl and php-symfony-polyfill-intl-icu by removing the dependency upon the polyfill.

Closes: #821138

---

I prepared this on the buster branch since that's what debcheckout gave me - is that the correct branch?