Rename pyproject_toml in dependency_getter to poetry

[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

In prevision of supporting #109, this PR renames pyproject_toml to poetry in dependency_getter, refactoring a bit the methods and tests along the way.

[codecov-commenter]

Codecov Report

Merging #151 (1a65052) into main (e71dc6a) will decrease coverage by 0.0%.
The diff coverage is 90.0%.

@@           Coverage Diff           @@
##            main    #151     +/-   ##
=======================================
- Coverage   92.2%   92.1%   -0.1%     
=======================================
  Files         42      42             
  Lines       1348    1346      -2     
=======================================
- Hits        1243    1241      -2     
  Misses       105     105             

Impacted Files	Coverage Δ
tests/dependency_getter/test_requirements_txt.py	100.0% <ø> (ø)
tests/issue_finders/test_missing.py	100.0% <ø> (ø)
tests/issue_finders/test_obsolete.py	100.0% <ø> (ø)
tests/issue_finders/test_transitive.py	100.0% <ø> (ø)
deptry/core.py	57.1% <33.3%> (ø)
deptry/dependency_getter/poetry.py	100.0% <100.0%> (ø)
tests/dependency_getter/test_poetry.py	100.0% <100.0%> (ø)

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[fpgmaas]

Good idea to rename it!