Problem/Motivation
As per the patch in #2090115: Don't install a module when its default configuration has unmet dependencies, it seems that locale was not modified, so it uses a default instantiated InstallStorage which defaults to using config/install only to discover default configuration. That means that optional configuration cannot be translated with locale. Either this needs to be resolved, or #2428045: Comparing active storage to install storage is problematic, install storage may change anytime needs to be resolved.
Proposed resolution
1. Remove the install storage's service entry (its not used other than tests).
2. Use instantiated versions of install storage for the default install dir and the optional dir in locale.
3. Add tests with an optional config.
Remaining tasks
Add tests. Review.
User interface changes
None.
API changes
config.storage.installer service is removed. The class used for the service is intact and not modified.