Fix indexing with compressed APT Packages files

APT can be configured to keep its Packages files compressed after downloading, and these days this is even a common setup. Therefore, one cannot assume that *Packages files under /var/lib/apt/lists/ exist, or can be read as plain text.

Instead, use the documented APT interfaces for listing and getting the contents of the Packages files, that work whether they are compressed or not.

Closes: #617856