Remove references to the kopia package from
`pkg/store` package so that kopia can import
that package itself. Do this by using
interfaces where needed in `pkg/store`
instead of concrete struct types
These changes will make cleaning up
incomplete backups a little neater since
that code will need to lookup both
manifests and backup models
This PR is just minor renaming and fixups,
no logic changes
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#### 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
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup
#### Issue(s)
* #3217
#### Test Plan
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [x] 💚 E2E
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