Ideas for tracking upstream references

Will be used to detect unused code (no upstream references) and upstream call hierarchy

* When index code, add an upstream reference to any invoked methods (regardless if inside or outside of method)
* When delete code documents, such as when reindexing file, also delete any upstream references to the deleted documents / code (will need to have something can search on, like method signature
* Then, when readd, can create document mentioning the upstream

Basically, at the same time I create a downstream reference, I'd also create an upstream reference to the current method / field / class

This way, can incrementally build upstream references.

How to easily identify methods / fields / classes with no upstream references (unused code)?
* Goal is that if I'm in a method, I want it to tell me the code isn't used
* That way, I'm not modifying unused code

Start by clarifying how indexing, deletion, and re-adding code documents would represent upstream references alongside downstream references. Define how method, field, and class signatures are identified and how the unused-code and call-hierarchy results should be surfaced before locating the relevant analyzer components.

Written by the indexing model from the issue text.