(drupal8-clean) The JS/CSS assets are sync'd to web dir, but the...

(drupal8-clean) The JS/CSS assets are sync'd to web dir, but the $civicrm_paths should still autoload PHP from the src folder.

This improves support for drupal8-clean:

1. The content of `$civicrm_paths['civicrm.packages']['path']` should be the
   *source*, because that has all files (including PHP and JS/CSS)
2. Fix generation of quasi-relative paths