Move modular repos to a subpackage
    
    We discovered this morning that PackageKit (a key component of GNOME
    Software and therefore the Fedora Workstation experience) does not
    handle the modular repository correctly. When DNF (and KDE's
    dnfdragora) interact with the repos, they see that there is modular
    metadata in play and they do not include it in the transaction
    dependency solving unless the module has been "enabled".
    
    However, PackageKit simply treats those repositories as if they were
    a standard repo, and thus any package in them with a higher ENVR
    than is in the traditional repo will depsolve at higher priority.
    
    We don't have sufficient time in the Fedora 28 schedule to fix this
    behavior properly in PackageKit (it will likely require moving much
    of the logic out of DNF and into a common layer such as libdnf), so
    in the interrim, we will make the modular repos a subpackage that
    Workstation Edition can opt not to ship by default.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>