Composites that return another transforms output are overridden incorrectly

The TransformHierarchy considers any transform that returns another transform's output to be a "composite". A.K.A. not a primitive. A primitive produces its own output.

But a composite that returns its input and _also_ feeds that input to an internal transform is rewired incorrectly. Because the output is replaced with the original, causing the internal PCollection to be incorrect.

Imported from Jira [BEAM-12228](https://issues.apache.org/jira/browse/BEAM-12228). Original Jira may contain additional context.
Reported by: kenn.