chore: Refactor sg migration (!30249) · Merge requests · Administrator / sourcegraph · GitLab

Administrator requested to merge ef/refactor-sg-migration into main Jan 26, 2022

Created by: efritz

Pulled from #29831. Should be behavior-preserving.

Collapsed squash and migration packages
Renamed Run -> Squash and RunAdd -> Add for consistency
Unexported util functions
Moved shared functions into util package
Reorder code blocks by use