If the loaded command from the entry_point already exists in the
main swh command group, do not add is as a command of this later.
The idea is if you have a swh package (say swh.objstorage) that
declares a cli group, then a sub-package of this package (say
swh.objstorage.replayer) also declares a cli group that is a
sub-group of the first package's one (swh.objstorage in the
example above), then we do not want to re-add it again.
Typically, if the entrypoint for swh.objstorage.replayer is:
with swh.objstorage.replayer.cli:objstorage_group being the cli
group of swh.objstorage, then we do not want to re-add this under
the main swh command group as 'content-replayer'.
Having it loaded is enought to make it registered under its parent
group, here, under swh objstorage.
Depends on D3901.