mapOutputs doesn't permit one replacement PValue to be used to represent multiple original PValues

It's not forbidden for a runner to replace outputs in this manner, but not currently possible based on the type signatures (PValue is a key into the map, and thus cannot be duplicated)

Imported from Jira [BEAM-1955](https://issues.apache.org/jira/browse/BEAM-1955). Original Jira may contain additional context.
Reported by: tgroh.

Start by locating the mapOutputs type signatures and review the linked BEAM-1955 Jira issue for additional context. Define the API change needed so one replacement PValue can represent multiple original PValues, then verify that the resulting types support this case without ambiguity.

Written by the indexing model from the issue text.