various tidbits of data cleanup before moving forward with adding export behavior to groups.
* move duplicate collections mocks into data/mock
* move the export collection struct into pkg/export (to prevent future duplicates in the next PR)
* rename export.Collection to Collectioner, because it's an interface.
* some other non-logic rearrangement
---
#### Does this PR need a docs update or release note?
- [x] ⛔ No
#### Type of change
- [x] 🧹 Tech Debt/Cleanup
#### Issue(s)
* #3991
#### Test Plan
- [x] ⚡ Unit test
- [x] 💚 E2E
9a8c413b52