Refactored issues and issue finders to include their error codes more clearly

[fpgmaas]

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

• Moved issue definitions and issue finders to the same directory, since they are linked.
• Added error codes to directory and class names. I think that makes the code more clear, especially if we add more error codes (like the one proposed in Potential DEP005: Detect Poetry extras that are not listed under dependencies. #400) in the future, and if we make the error codes play a more important role (as proposed for example in Replace --ignore-* flags with select (and optionally ignore.) #398).

Curious for your thoughts!

[codecov]

Codecov Report

Merging #401 (7c49c2f) into main (3621144) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #401     +/-   ##
=======================================
- Coverage   96.1%   96.1%   -0.1%     
=======================================
  Files         38      37      -1     
  Lines       1072    1070      -2     
  Branches     215     215             
=======================================
- Hits        1031    1029      -2     
  Misses        25      25             
  Partials      16      16             

Impacted Files	Coverage Δ
deptry/core.py	98.0% <100.0%> (-0.1%)	⬇️
deptry/violations/__init__.py	100.0% <100.0%> (ø)
deptry/violations/base.py	100.0% <100.0%> (ø)
deptry/violations/dep001_missing/finder.py	100.0% <100.0%> (ø)
deptry/violations/dep001_missing/violation.py	100.0% <100.0%> (ø)
deptry/violations/dep002_unused/finder.py	93.5% <100.0%> (ø)
deptry/violations/dep002_unused/violation.py	100.0% <100.0%> (ø)
deptry/violations/dep003_transitive/finder.py	88.4% <100.0%> (ø)
deptry/violations/dep003_transitive/violation.py	100.0% <100.0%> (ø)
deptry/violations/dep004_misplaced_dev/finder.py	89.1% <100.0%> (ø)
... and 1 more