Fix file lists on packages.d.o. (Closes: #977006, #977743, #980888, #923974)

Concatenate results from Contents-${arch}.gz and Contents-all.gz when the latter is present.

With this commit in daklib, the archive started to provide separate Contents files with packages from Arch:all for some suites. (All suites post-buster appear to be affected.) Those file lists were not scanned for the web pages powering packages.d.o, and the file lists were unavailable online.

This commit attempts to resolve the issue by parsing the file for Arch:all immediately afterwards, if it is present. The results are effectively concatenated.

A better long-term solution might be to produce separate transfer files for Arch:all, but that may not work until buster is dropped from the archive.

This merge request was tested on picconi.d.o. In the view of the requestor, the second commit in this series should not have been needed. For a comprehensive explanation and two caveats, please have a look at the bug amendment here or here.

The mysterious second commit in this may fix Bug#923974. I have a feeling the file lists were not being updated at all. This merge request should restore general service.

Before merging this, please urge the Salsa Team to accept another open merge request over there. That MR fixes a long-standing webhook malfunction that surfaces when closing multiple bugs. Like this MR, it was tested.