Fix optional config in profiles overriding shipped module configuration being ignored

Problem/Motivation

Due to the way we use the union storage optional configuration is always checked last, regardless of its origin. This is problematic if a profile wants to override shipped configuration of a module, but ship that override in its own config/optional directory, because it does not directly depend on the module. With Config Overlay the module-provided config will be read instead of the profile-provided one.

Steps to reproduce

1. Have an installation profile that does not install announcements_feeds out of the box, but provides a modified announcement_feeds.settings configuration in config/optional, for example to set the limit from 10 to 5.
2. Install the profile and subsequently install announcements_feeds. Observe that the limit is 5.
3. Now install Config Overlay, export configuration and import configuration. Observer that the import will incorrectly attempt to change the limit back to 10.