Some apport-retrace refactoring

[schopin-pro]

A couple of commits that split out some code out of apport-retrace's main() and into helper functions.

Depends on #295

[schopin-pro]

Marking as Draft since there are no tests to back it up.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (69e81cf) 81.86% compared to head (7418eb2) 81.86%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #297   +/-   ##
=======================================
  Coverage   81.86%   81.86%           
=======================================
  Files          93       93           
  Lines       19030    19030           
  Branches     3047     3047           
=======================================
  Hits        15578    15578           
  Misses       2949     2949           
  Partials      503      503           

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