repo-updater: Refactor Store scanning and pagination

Created by: tsenart

In preparation for extending Store with ListSources and UpsertSources, which is required for upcoming migrations, this commit refactors internal methods of pagination and scanning to leverage closure side effects for polymorphism.

This is a more type-safe approach than refactoring to use *[]interface{} instead of *[]*Repo.

Part of #2025 (closed)