perf(core): only load local modules once

[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

Local modules retrieval is currently done multiple times, as it's done in a for loop. This PR makes sure that it's only done once, greatly reducing the run time on big projects.

[codecov-commenter]

Codecov Report

Merging #242 (8b5ad30) into main (8a71695) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #242   +/-   ##
=====================================
  Coverage   95.7%   95.7%           
=====================================
  Files         32      32           
  Lines        914     915    +1     
  Branches     203     203           
=====================================
+ Hits         875     876    +1     
  Misses        26      26           
  Partials      13      13           

Impacted Files	Coverage Δ
deptry/core.py	97.6% <100.0%> (+<0.1%)	⬆️

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