Comment 25 for bug 1821251

TEST CASE:
1. Installed current non-SRU package: apt install debmirror=1:2.27ubuntu1
2. Created a 20.04 Focal mirror: /usr/bin/debmirror -a amd64,i386 -d focal,focal-security,focal-updates,focal-backports -s main,universe,restricted,multiverse --nosource --passive --method=http -h archive.ubuntu.com -r ubuntu --keyring=/usr/share/keyrings/ubuntu-archive-keyring.gpg --progress /opt/debmirror
3. Exposed /opt/debmirror as webroot on test.example.com
4. On a separate node, configured APT to use the mirror, via sources.list such as:
   deb http://test.example.com/ubuntu/ focal main universe restricted multiverse
   deb http://test.example.com/ubuntu/ focal-security main universe restricted multiverse
   deb http://test.example.com/ubuntu/ focal-updates main universe restricted multiverse
5. Tried to use mirror on the newly configured client: apt update
   E: Failed to fetch http://test.example.com/ubuntu/dists/focal/main/cnf/Commands-amd64 404 Not Found [IP: 10.10.10.10 80]
   E: Failed to fetch http://test.example.com/ubuntu/dists/focal-security/main/cnf/Commands-amd64 404 Not Found [IP: 10.10.10.10 80]
   E: Failed to fetch http://test.example.com/ubuntu/dists/focal-updates/main/cnf/Commands-amd64 404 Not Found [IP: 10.10.10.10 80]
   E: Some index files failed to download. They have been ignored, or old ones used instead.
6. Installed SRU package: apt install debmirror=1:2.27ubuntu2.20.04.1
7. Repeated `debmirror` invocation from step 2, in order to pull in cnf files.
8. Repeated `apt update` invocation as from step 5, in order to try using the mirror.

VERIFICATION DONE
APT client succeeds, and mirrored repository can be successfully used.