Show total number of items in sync-tasks

Is your feature request related to a problem? Please describe.
pulp_deb progress_reports during sync tasks should show the total number of items to process. Currently it just says null:

"progress_reports": [
        {
            "code": "update.release_file",
            "done": 0,
            "message": "Update ReleaseFile units",
            "state": "running",
            "suffix": null,
            "total": null
        },
        {
            "code": "update.packageindex",
            "done": 10,
            "message": "Update PackageIndex units",
            "state": "running",
            "suffix": null,
            "total": null
        },
        {
            "code": "associating.content",
            "done": 544,
            "message": "Associating Content",
            "state": "running",
            "suffix": null,
            "total": null
        },
        {
            "code": "sync.downloading.artifacts",
            "done": 938,
            "message": "Downloading Artifacts",
            "state": "running",
            "suffix": null,
            "total": null
        }
    ]

Describe the solution you'd like
I reckon this could mean iterating over the PackageIndex before actually processing it, maybe a 'nicer' solution is possible.
Nevertheless, it would make the current state of the task more transparent.

Start by tracing how pulp_deb sync tasks build their progress_reports, especially the PackageIndex processing path and the reported total fields. Determine where the item count can be obtained before processing; done means the relevant progress reports expose a populated total instead of null.