[Improvement] See results when symbols are only used in overrides

Love the plugin, thanks so much for building it!

**Suggestion for improvement**
As a user I'd like to see the instances a symbol is used as a symbol override as well as the symbol placements.

For example, in a document there are multiple symbols. They are not all used on an artboard literally, but some appear because they've been selected in a symbol override.

No file, test, or entry point is named in the issue. Start by locating how the plugin finds symbol placements and compare that with symbols selected through overrides; done means results include both literal placements and override instances.

Written by the indexing model from the issue text.