Fix lp copy packages arbitrary lookback (infra)

[Hook25]

Description

The lp_copy_packages.py script failed to update our stable PPA because it assumed that any package that had to be copied over was less than 4 weeks old.

This is false for two reasons:

1. A package has to be copied over regardless of how old it is if it is still the most recent version we have (namely, it took us way more than 1month to release stable this time around)
2. A package doesn't have to be copied over if it is Superseeded (namely, we have a more current version in the PPA of that same package)

Note: there is a bit more insight on the filter I have left in the comment, but this is the top level view.
Note1: the old comment stating that LP will choke on the request if no date is provided is only true if the iterator that the request returns is fully unwrapped. For this reason this PR only unwraps the relevant part.

Resolved issues

Last stable release failed to update stable channel, see log: https://github.com/canonical/checkbox/actions/runs/7987143879/job/21809020771

(No copying over message -> no package copied over)

Documentation

This PR adds documentation for what the function does and also adds a note about the more "counterintuitive" side of it (the fact that non-superseeded packages may still be non-copy-wrothy)

Tests

This updates the test with the new two cases at hand

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 40.92%. Comparing base (539ec18) to head (08cad09).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1067   +/-   ##
=======================================
  Coverage   40.92%   40.92%           
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  Files         337      337           
  Lines       37547    37548    +1     
  Branches     6383     6384    +1     
=======================================
+ Hits        15365    15366    +1     
  Misses      21543    21543           
  Partials      639      639           

Flag	Coverage Δ
release-tools	74.32% <100.00%> (+0.06%)	⬆️

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[fernando79513]

LGTM +1