add switch to filter out meta packages

Created by: alpianon

In some harvest.json files also meta-packages are included (i.e. packages with no source files whatsoever, that have just deps in order to force the installation of a group of other packages), but since they are not "real" packages, a filter should be added in order to filter them out and get only "real" packages displayed.

See for example this harvest.json file in the dashboard and filter for "packagegroup".

Meta packages can be distinguished because they have 0 total files:

      "statistics": {
        "files": {
          "audit_total": 0,
          "audit_done": 0,
          "audit_to_do": 0,
          "upstream_source_total": 0,
          "unknown_provenance": 0,
          "known_provenance": 0,
          "total": 0
        },