Add support for deb822 format for setting default MIRRORSITE

Michael Prokoprequested to merge
mika/pbuilder:mika/deb822 into master

In more recent Debian releases the deb822 format is used for apt sources.list configuration (see sources.list(5) for details).

The traditional format ("one-line-style format") for e.g. /etc/apt/sources.list or /etc/apt/sources.list.d/debian.list looks like this:

  % cat /etc/apt/sources.list
  # deb http://snapshot.debian.org/archive/debian/20250811T000000Z bullseye main
  deb http://deb.debian.org/debian bullseye main
  # deb http://snapshot.debian.org/archive/debian-security/20250811T000000Z bullseye-security main
  deb http://deb.debian.org/debian-security bullseye-security main
  # deb http://snapshot.debian.org/archive/debian/20250811T000000Z bullseye-updates main
  deb http://deb.debian.org/debian bullseye-updates main

With deb822 we instead get something like:

  % cat /etc/apt/sources.list.d/debian.sources
  Types: deb
  # http://snapshot.debian.org/archive/debian/20250929T000000Z
  URIs: http://deb.debian.org/debian
  Suites: bookworm bookworm-updates
  Components: main
  Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg

  Types: deb
  # http://snapshot.debian.org/archive/debian-security/20250929T000000Z
  URIs: http://deb.debian.org/debian-security
  Suites: bookworm-security
  Components: main
  Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg

Support both file formats at the same time to identify the default MIRRORSITE for usage in /etc/pbuilderrc.

Closes: #1032153

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