critpath.py: also record critpath by groups in a JSON file
    
    This makes critpath.py output a JSON file containing a record
    of the calculated critical path for each individual critpath
    group, as well as the flat text file containing the entire
    combined critical path. The ultimate goal is to move Bodhi over
    to tagging updates as critical path by group and also doing
    critical path gating queries per group, to give testers more
    information on exactly what an update is "critical" for, and
    allow more flexibility in scheduling automated tests and gating.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>