apt: Introduce the modernize-sources command

This handles rewriting .list entries to .sources ones; the
only options handled are

- signed-by => Signed-By
- trusted => Trusted

The options to specify architectures, check-valid-until, etc.
are not supported as they are not exposed in the code. We
probably should refactor the parser at some point so we can
actually read the parsed entry rather than the abstract objects
we create from it.

This can be improved further:

- Keyrings from /etc/apt/trusted.gpg.d should be moved to
  /etc/apt/keyrings.
- automatic detection by trying to verify using each keyring
  in trusted.gpg.d until we find the right one and then using
  that for Signed-By rather than leave some sources empty.

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