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>