Avoid term lookups when merging dependencies

[charliermarsh]

Summary

The merged-dependency hash index narrows each candidate search to incompatibilities with the same dependent package, dependency package, and dependency-range hash. merge_dependents then reconstructs those same ranges through four SmallMap lookups and positive/negative Term unwrapping before it can compare and merge them.

This change destructures the two FromDependencyOf kinds once, compares their stored dependency ranges directly to retain collision safety, and unions their stored dependent ranges. The self-dependency and package-identity guards are unchanged. The old negative-term unwrapping helper becomes unused and is removed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing charlie/direct-dependency-merge (6dd7ca3) with main (cfe9498)}