epm-downgrade: restore suddenly removed code for full downgrade (ALT bug 47048)

bin/epm-downgrade

@@ -104,6 +104,11 @@ epm_downgrade()
                 pkgs="$pkgs $i"
             done
             (force="$force --oldpackage" epm_install_files $pkgs)
+        else
+            __epm_add_alt_apt_downgrade_preferences || return
+            load_helper epm-upgrade
+            epm_upgrade "$@"
+            __epm_remove_apt_downgrade_preferences
         fi
         return
         ;;