[swdb] Put Swdb constructors together
[swdb] Put Swdb constructors together

It confused me when reading this code that the `Swdb` constructors
weren't all together.

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