feat: add ability to pass multiple source directories

[mkniewallner]

Resolves #177.

Note: This PR is based on #380, so first commit is not part of this PR.

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

While not that common, some projects have multiple source directories, for instance if they define code used inside a web server in a dedicated directory, and code used in a dedicated Kubernetes job like a CRON in another dedicated directory.

In order to give more flexibility to developers, this PR updates ROOT argument so that multiple paths can be passed on the CLI, like so:

deptry a_directory another_directory

This won't break existing usages, since developers can still pass one directory as they are used to.

[codecov]

Codecov Report

Merging #381 (ce46162) into main (640d45b) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #381   +/-   ##
=====================================
  Coverage   96.0%   96.0%           
=====================================
  Files         37      37           
  Lines       1042    1044    +2     
  Branches     207     208    +1     
=====================================
+ Hits        1001    1003    +2     
  Misses        25      25           
  Partials      16      16           

Impacted Files	Coverage Δ
deptry/cli.py	91.6% <100.0%> (ø)
deptry/core.py	98.0% <100.0%> (ø)
deptry/python_file_finder.py	100.0% <100.0%> (ø)