establishes behavior around using the reasoner interface
in a world where paths can contain multiple services. Primarily
focused on ensuring the reasoner clearly guides maintainers
towards proper usage.
Moves the kopia.Reasoner interface out of the
kopia package and into pkg/backup/identity.
No logical changes here, just renaming and movement.
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#### Does this PR need a docs update or release note?
- [x] ⛔ No
#### Type of change
- [x] 🧹 Tech Debt/Cleanup
#### Issue(s)
* #3993
#### Test Plan
- [x] ⚡ Unit test
- [x] 💚 E2E