When gather_method is set to nodeps, we should not ignore the comps
group that the method received. Instead it should find out which
packages are in those groups and take them into the compose.
In order for this to be of any reasonable use, the comps file needs to
include all dependencies for the packages.
Fixes: #653
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>