Geo: Enumerate project_repositories instead of projects for verification/replication of Git repos

This is part of a multi-step implementation plan to handle Geo verification/replication of projects without Git repositories to reduce errors caused when Gitaly tries to fetch non-existent repos.

Like Design Management Repositories, we want Geo to enumerate project_repositories table instead of the projects table, because a ProjectRepository contains a Git repo, not a Project itself (in the gitlab rails code).

This will simplify and our logic for determining when a Git repo needs to be fetched for verification/replication.