Enforce all inputs and outputs are properly declared in Transform Hierarchy

Within a composite transform, each child node may only consume values declared as the input to the composite and values produced as output by other direct children of the composite node. This can be enforced automatically via pipeline traversal, or by the `TransformHierarchy` directly.

Imported from Jira [BEAM-2428](https://issues.apache.org/jira/browse/BEAM-2428). Original Jira may contain additional context.
Reported by: tgroh.

Start by reading TransformHierarchy and the pipeline traversal behavior described in the issue. Determine how composite children declare consumed and produced values, then enforce that each child uses only the composite input or direct-child outputs. Done means invalid undeclared inputs or outputs are rejected automatically.

Written by the indexing model from the issue text.