Generated main/debug/<arch>/Packages.xz/gz files are empty

According to the documentation: https://wiki.ubuntu.com/DebuggingProgramCrash#Installing_dbgsym_packages_from_a_PPA in order to enable a repository's debug symbols section, I have to add main/debug.

I did so for many, many repos, but here's the result for Firefox:

```
deb http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu bionic main main/debug
deb-src http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu bionic main
```

then we run apt update with the following result:

All packages are up to date.
W: Skipping acquire of configured file 'main/debug/i18n/Translation-en' as repository 'http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu bionic InRelease' doesn't have the component 'main/debug' (component misspelt in sources.list?)
W: Skipping acquire of configured file 'main/debug/i18n/Translation-en_US' as repository 'http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu bionic InRelease' doesn't have the component 'main/debug' (component misspelt in sources.list?)
W: Skipping acquire of configured file 'main/debug/dep11/Components-amd64.yml' as repository 'http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu bionic InRelease' doesn't have the component 'main/debug' (component misspelt in sources.list?)
W: Skipping acquire of configured file 'main/debug/Contents-i386' as repository 'http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu bionic InRelease' doesn't have the component 'main/debug' (component misspelt in sources.list?)
W: Skipping acquire of configured file 'main/debug/Contents-amd64' as repository 'http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu bionic InRelease' doesn't have the component 'main/debug' (component misspelt in sources.list?)
W: Skipping acquire of configured file 'main/debug/cnf/Commands-amd64' as repository 'http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu bionic InRelease' doesn't have the component 'main/debug' (component misspelt in sources.list?)

We get the warnings for all these missing files, but we don't get a warning that the downloaded Packages.xz is empty!

So even tho the -dbgsym package exists: http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu/pool/main/f/firefox/
it's not referenced anywhere and so cannot be installed.