Description of problem:
I tried to install some debug symbols on rawhide and it failed.

Version-Release number of selected component (if applicable):
sh# rpm -q dnf dnf-plugins-core 
dnf-2.6.3-2.fc28.noarch
dnf-plugins-core-2.1.3-2.fc27.noarch

How reproducible:
Deterministic

Steps to Reproduce:
1. docker run -ti --rm fedora:rawhide bash
   //run rest of commands in container
2. dnf install -y --best --nogpgcheck 'dnf-command(debuginfo-install)'
3. rpm -q dnf dnf-plugins-core
4. dnf debuginfo-install libdnf

Actual results:
sh# rpm -q dnf dnf-plugins-core 
dnf-2.6.3-2.fc28.noarch
dnf-plugins-core-2.1.3-2.fc27.noarch

sh# dnf debuginfo-install libdnf
No such command: debuginfo-install. Please use /usr/bin/dnf --help
It could be a DNF plugin command, try: "dnf install 'dnf-command(debuginfo-install)'"

Expected results:
debug info symbols installed without any problem