dependencies api: Prepare for batch repository sync calls
Created by: efritz
Stacked on #31870. This PR does some of the work required to be able to send a single (or, at a minimum, batched) requests to repo-updater. To make use of this in the future:
- codeintel side should send larger batches instead of
n
calls with singleton lists - search side should update repo-updater interface instead of looping over them and making individual requests
Test plan
Behavior-preserving refactor.