Paths to the interface inputs are not returned from code generation

Currently code generation will return a path to a node input even if it's connected to an interface input.
This is being handled after the fact by patch code for instance in the Material Viewer.

It is thus possible to get / set the incorrect value and may miss colorspace or unit information.

Suggestion is to move patch this in code generation.

Adding @niklasharrysson and @jstone-lucasfilm for thoughts.

The issue names code generation and patch code in the Material Viewer but no files or tests. Start by tracing how paths to node inputs are generated and how the Material Viewer patches interface inputs; done means generated paths resolve to the interface input and preserve colorspace and unit information.

Written by the indexing model from the issue text.