refactor(dependency_getter): extract requirements files extraction

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added or modified a feature, documentation in docs is updated

Description of changes

Extracted from #881. This makes the extraction of dynamic dependencies from setuptools cleaner than having to instantiate RequirementsTxtDependencyGetter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.4%. Comparing base (3fc6405) to head (3cbf622).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #886     +/-   ##
=======================================
+ Coverage   92.1%   92.4%   +0.2%     
=======================================
  Files         37      37             
  Lines        992     989      -3     
  Branches      96      96             
=======================================
  Hits         914     914             
+ Misses        62      60      -2     
+ Partials      16      15      -1     

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