[Visualizer] create link between original resource and reference resource when using 'existing' key word

Hi,

New visualizer is really cool, nice work, just one thing I spotted that made my graph then look strange.

**Is your feature request related to a problem? Please describe.**
I create resource reference using existing key word in other module files but the visualizer doesn't show a link back to where the original resource was created

**Describe the solution you'd like**
Would be nice if using the 'existing' key word would show link back to original resource on the visualizer

The issue names the visualizer and the 'existing' keyword but no files, tests, or entry points. Start by locating the visualizer implementation and the handling of references created with 'existing'. Done means the visualizer shows a link from each reference resource back to the original resource declaration.

Written by the indexing model from the issue text.