The problem, for anyone else that might encounter it, involves the fact that the …\packages\WmPartners\code\classes\wm\PartnerMgr directory contains both an xtn class and an xtn package. From the compiler’s perspective, the class hides the package, so the classes in the package are unable to be resolved.
I solved the problem by temporarily renaming the xtn.class file while I compile the service that needs to access the xtn package. Tedious, but it seems to work.
#webMethods#B2B-Integration#Integration-Server-and-ESB