report: rename varlink dir for upload plugins

report: rename varlink dir for upload plugins

let's name the dir "/run/systemd/report.upload/" (rather than
"/run/systemd/metrics-upload/"). After all, these are reports that we
upload, not indiviudual metrics. And it would be particular confusing
since the dir to pick up metrics is called /run/systemd/report/, rather
than /run/systemd/metrics/. Hence the thing that deals with reports is
nmamed metrics, and the thing that deals in metrics is named reports...