Optimize /api/v4/projects endpoint for visibility level

Previously when a user requested a list of projects, Project#public_or_visible_to_user would search all authorized projects and public/internal projects as well. However, when a user requests a specific visibility_level (e.g. private), that should reduce the search space, and we shouldn't need to load public/internal projects.

Improves https://gitlab.com/gitlab-org/gitlab-ce/issues/59329