Index SBOM occurrences for parent lookup in graph build

Ryan Wellsrequested to merge
rw/593156-sbom-occurrence-index into master

What does this MR do and why?

find_parent_sbom_occurrence scanned every occurrence for each ancestor, which is quadratic in the number of occurrences for large projects.

This change introduces an in-memory index to make lookup O(1).

Related to #593156

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