The org.apache.sis.referencing package contains a DefaultReferencingIdentifier class implementing the GeoAPI ReferencingIdentifier interface. This class was used to be defined in the referencing module, but we had to move it to the metadata module because of dependencies at XML (un)marshalling time (the JAXB adapters needs a ReferencingIdentifier implementation).
This is fine in a non-OSGi world. But in a OSGi context, we can not share the same package between two different modules. So we need to move that class elsewhere. One possibility is to put this class in the same metadata package than DefaultIdentifier and rename it as ImmutableIdentifier to better stress out that this; implementation is immutable, as opposed to DefaultIdentifier.