`add-apt-repository --pocket` doesn't work on Noble (Deb822?)

[Ratchanan Srirattanamet]

It seems like `add-apt-repository` doesn't know how to enable a new pocket when Ubuntu's sources are defined in Deb822 (.sources) source file.

Step to reproduce:
1. Create a Noble chroot with e.g. Docker. Verify that in such chroot, Deb822 is used for Ubuntu's source (`ls /etc/apt/sources.list.d/ubuntu.sources`).
2. Run `add-apt-repository --pocket proposed`. Accept the prompt.
3. Run `apt policy`.
  - Expected result: `noble-proposed` is listed.
  - Actual result: `noble-proposed` is not listed.

This might be related to LP: #2060331, but since I'm not sure how it works, I'm filing a new bug here.

ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: software-properties-common 0.99.46
ProcVersionSignature: Ubuntu 6.5.0-27.28~22.04.1-generic 6.5.13
Uname: Linux 6.5.0-27-generic x86_64
ApportVersion: 2.28.0-0ubuntu1
Architecture: amd64
CasperMD5CheckResult: unknown
CloudSerial: 20240225
Date: Fri Apr 12 18:38:12 2024
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, no user)
 TERM=xterm
SourcePackage: software-properties
UpgradeStatus: No upgrade log present (probably fresh install)

[Nick Rosbrook (enr0n)]

Hm, yeah this was overlooked too. The initial deb822 support in software-properties was mostly about adding PPAs. It looks like we need to go through and add more general deb822 support.