Custom CSS URL not working if aliased or rewritten

The Custom CSS URL is not working if this URL is defined as an Alias or going through a rewrite rule in the Apache configuration.

For example, often people alias the /files URL to directory /sites/default/files on disk to have shorter and more readable URLs.

This is due to the fact that the CRM_Utils_System_DrupalBase::formatResourceUrl function "optimizes" the URLs by rewriting them as included files so Drupal can aggregate CSS files. However it does not test whether the files exists in the file system before doing so.