Refactor: Rely on a list when checking service URL availability

Extracted from !517 (closed)

Previously in serviceCatalogToEndpoints we have been converting a list of service names and computed URLs into a Dict and performing various computations on that Dict to determine if all are availabe, and if not, which are missing.

This accidentally created an abstraction overload where we were doing more work than is required by the initial data structure, the List.

In this patch we're removing the Dict and relying on list operations, importantly List.filterMap, which maps values and if they are Nothing will remove them from the resulting list. This simplifies the logic and avoids a superfluous level of abstraction.