Discussion: tool for noticing difference between BOM and explicit dependencies

Customer issues in b/146130570 are making me think it might be too hard in large projects that extend past GCP to tell which versions can and cannot be set by the BOM. Two ideas:

1. Docs listing what's covered in the BOM.
2. An automated tool to suggest changes to a pom.xml when a dependency has an explicit version that is also set by the BOM.

Perhaps an enforcer rule could warn about #2?

**DO NOT START IMPLEMENTING THIS. IT HAS NOT BEEN PRIORITIZED OR PLANNED. THIS IS FOR DISCUSSION ONLY**

This issue is explicitly discussion-only and says not to start implementation. If it is later prioritized, begin by reviewing the BOM coverage and the pom.xml dependency-version cases described here, then decide whether the deliverable is documentation, an automated suggestion tool, or an enforcer warning; no tests or implementation entry points are named.

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