With this patch, there should be a separate package set for each tag
that is consumed.
Generally each module will create a separate package set, with the
exception of -devel modules that will be in the same set as their
non-devel version.
Variants no longer need to keep their own package set objects. Instead
they now include a set of package set names that should be used for the
variant. This can replace the whitelist mechanism of deps gather method.
JIRA: COMPOSE-3620
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>