refactor: simplify dependency specification detector

[mkniewallner]

Note

Review commit by commit is advised.

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!
• Documentation in docs is updated

Description of changes

This is a preliminary refactoring for #641. I figured that we could simplify a bit the logic in the dependency specification detector by directly returning instances of DependencyGetter instead of an enum that is then handled in Core class.

This should make overriding requirements_files with requirements.in if detected a bit cleaner.

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 91.8%. Comparing base (b8fd0e3) to head (597ad11).

Files	Patch %	Lines
python/deptry/core.py	50.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #654     +/-   ##
=======================================
+ Coverage   90.6%   91.8%   +1.2%     
=======================================
  Files         34      34             
  Lines        961     947     -14     
  Branches     195     197      +2     
=======================================
- Hits         871     870      -1     
+ Misses        73      60     -13     
  Partials      17      17             

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