feat(visitor): skip type checking blocks

[mkniewallner]

Resolves #556.

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

When guarding imports with TYPE_CHECKING in combination with from __future__ import annotations, they are not evaluated at runtime (https://peps.python.org/pep-0563/). This PR implements that behavior, by completely skipping those imports in the import visitor.

An alternative implementation could be to flag dependencies as "typing only" instead of skipping them. This would allow deptry to trigger DEP001 (missing dependencies).

This would also avoid having deptry trigger DEP002 if a dependency is only used for typing, but for this one IMO, it's actually a good thing that it would report a violation, as the dependency is not necessary at runtime, so it should not be a non-dev direct dependency. Still, this is something that could be weird for users that encounter such issues. I've added a new documentation section that details how imports are extracted to at least explain the mechanism used.

I'm not really sure that the extra burden of flagging the dependencies instead of completely skipping them is worth it, but let me know if you think otherwise, I'd be ok with updating the implementation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.6%. Comparing base (dba1b70) to head (5d4ad37).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #652   +/-   ##
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  Coverage   90.6%   90.6%           
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  Files         34      34           
  Lines        961     961           
  Branches     195     195           
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  Hits         871     871           
  Misses        73      73           
  Partials      17      17           

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