## Description
Have BackupOp produce the paths for relevant subtrees in each snapshot and pass those to BackupCollections. This removes the need for code in the kopia package to call into more service/category-specific path package code, thus keeping the kopia package more generic.
As in #1828, prefix info for each subtree path is pulled from the Reason a snapshot was selected.
## Does this PR need a docs update or release note?
- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No
## Type of change
- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [ ] 🐹 Trivial/Minor
## Issue(s)
* closes#1832
## Test Plan
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E
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