refactor: improve violation finder

[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!
• Documentation in docs is updated

Description of changes

This PR reduces a bit code duplication and moves the violations finder logic out of Core god class.

[codecov]

Codecov Report

Attention: Patch coverage is 76.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 92.4%. Comparing base (a40f47b) to head (c2cd410).

Files	Patch %	Lines
python/deptry/violations/finder.py	61.5%	5 Missing ⚠️
python/deptry/core.py	50.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #655     +/-   ##
=======================================
+ Coverage   91.8%   92.4%   +0.5%     
=======================================
  Files         34      35      +1     
  Lines        947     950      +3     
  Branches     197     194      -3     
=======================================
+ Hits         870     878      +8     
+ Misses        60      55      -5     
  Partials      17      17             

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.