Use libdnf::Subject directly in hawkey.Subject
Use libdnf::Subject directly in hawkey.Subject

Use the new libdnf::Subject class directly instead of through
hy_subject_get_best_solution().

Closes: #795
Approved by: j-mracek

rpm-software-management/libhif,rpm-software-management/libdnf,hughsie/libhif,hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libhif,rpm-software-management/libhif