Rename UserConfiguration to Settings ...

The latter is more accurate, as we can have system settings
of type UserConfiguration.
This is also a push towards more consistent naming within dub,
as configuration is primarily used for package recipe 'configuration' field.

Move user settings to dub.data.settings ...

By moving it to its own module, we can ensure it's almost a leaf module.
We can also extend unittests to check different scenarios
without relying on integration test.