feat(core): handle local modules without __init__.py

[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

Since PEP 420, it is not required for Python modules to have an __init__.py to be considered a Python module.
We also today only consider directories and not Python files.

This PR handles this case by relaxing _get_local_modules method to account for 2 things:

• Directories that don't contain any __init__.py, by checking for any Python file
• Files that are Python files (end up with .py extension)

Tests have been updated to reflect what this implies for projects using an src directory structure.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@3c61806). Click here to learn what that means.
The diff coverage is 100.0%.

❗ Current head eeda478 differs from pull request most recent head f7cc6ca. Consider uploading reports for the commit f7cc6ca to get more accurate results

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  Coverage        ?   95.6%           
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  Files           ?      29           
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  Hits            ?     888           
  Misses          ?      27           
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Impacted Files	Coverage Δ
deptry/core.py	98.0% <100.0%> (ø)

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