Notebooks with cached stages via magic can cause conflicting outputs

If we declare an output path from a notebook and also in one of its cells via the stage magic we have a DVC conflict. We should only have the the output paths in one place.

Start by tracing how notebook-declared outputs and outputs declared through the stage magic are collected, then reproduce the conflict described in the issue with a notebook. Done means an output path declared in both places is handled consistently so the project has only one output-path declaration.

Written by the indexing model from the issue text.