repos: revert package-repositories changes for core18 and core20

[tigarmo]

This PR restores the original (pre-craft-archives) support for package-repositories for core18 and core20. This is done by reverting the changes done to the snapcraft_legacy/ tree (and its tests) in these two commits:

• 0e12894 (requirements: update craft-archives)
• ce23bc7 (repo: migrate to craft-archives)

[codecov-commenter]

Codecov Report

Merging #4279 (a3cff2a) into hotfix/7.5 (4d7eab2) will increase coverage by 0.02%.
The diff coverage is 96.75%.

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@@              Coverage Diff               @@
##           hotfix/7.5    #4279      +/-   ##
==============================================
+ Coverage       94.85%   94.88%   +0.02%     
==============================================
  Files             649      657       +8     
  Lines           54449    55116     +667     
==============================================
+ Hits            51650    52297     +647     
- Misses           2799     2819      +20     

Impacted Files	Coverage Δ
...apcraft_legacy/internal/meta/package_repository.py	90.00% <90.00%> (ø)
snapcraft_legacy/internal/repo/apt_ppa.py	90.90% <90.90%> (ø)
...pcraft_legacy/internal/repo/apt_sources_manager.py	95.40% <95.40%> (ø)
snapcraft_legacy/internal/repo/apt_key_manager.py	98.59% <98.59%> (ø)
snapcraft_legacy/internal/meta/snap.py	97.67% <100.00%> (ø)
...napcraft_legacy/internal/project_loader/_config.py	92.76% <100.00%> (ø)
snapcraft_legacy/plugins/v1/_plugin.py	95.91% <100.00%> (ø)
snapcraft_legacy/plugins/v1/catkin.py	93.99% <100.00%> (ø)
snapcraft_legacy/plugins/v1/colcon.py	98.30% <100.00%> (ø)
tests/legacy/unit/meta/test_package_repository.py	100.00% <100.00%> (ø)
... and 3 more

... and 1 file with indirect coverage changes

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