Remove unnecessary code in `PackagesListBuilder` + Refactor tests

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues	-

[what-the-diff[bot]]

PR Summary

• Deletion of Unwanted Files A file named 'merge-plan.php' was removed from our codebase. It's a cleanup effort that helps in maintaining code quality

• Modification of Test Configurations The 'ConfigTest.php' file was also updated, further hinting towards improvements in our code testing environment and configurations. This might lead to more reliable code testing.

[codecov[bot]]

Codecov Report

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

Comparison is base (f8288d9) 99.59% compared to head (b786e56) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #157 +/- ## ============================================= + Coverage 99.59% 100.00% +0.40% + Complexity 282 277 -5 ============================================= Files 22 22 Lines 736 728 -8 ============================================= - Hits 733 728 -5 + Misses 3 0 -3 ```

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