fix(python_file_finder): load gitignore from where CLI is invoked

[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

.gitignore is currently looked for based on the root directory passed as a CLI argument. This means that developers using a src directory and invoking deptry with deptry src won't benefit from gitignore being loaded.

I don't think that there are a lot of use cases where .gitignore would be located in a different directory that the one deptry is launched from, so this PR updates the behaviour so that .gitignore is always loaded based on where deptry is invoked.

If there really is a need for explicitly configuring the path to .gitignore, we can think about adding an option later.

Note that this PR will also facilitate the implementation of #381, even if doesn't strictly requires it, since we could also loop over each directory passed through the CLI.

[codecov]

Codecov Report

Merging #380 (fa3e44e) into main (18dad91) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #380   +/-   ##
=====================================
  Coverage   96.0%   96.0%           
=====================================
  Files         37      37           
  Lines       1042    1042           
  Branches     207     207           
=====================================
  Hits        1001    1001           
  Misses        25      25           
  Partials      16      16           

Impacted Files	Coverage Δ
deptry/python_file_finder.py	100.0% <100.0%> (ø)