Improve custom generator class discovery in load_generators method

[rszamszur]

Checklist:

• Added tests for changed code where applicable.
• Documentation reflects the changes where applicable.
• Updated the CHANGELOG.md file with your changes.
• My PR is ready to review.

Resolves: #137

Description of the changes being introduced by the pull request:

[github-actions]

🚀 Deployed on https://6303c706d2be6910585659e9--fastapi-mvc.netlify.app

[codecov-commenter]

Codecov Report

Merging #138 (00e81af) into master (4c93d8e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #138   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           40        40           
  Lines          801       810    +9     
=========================================
+ Hits           801       810    +9     

Impacted Files	Coverage Δ
fastapi_mvc/__init__.py	100.00% <100.00%> (ø)
fastapi_mvc/borg.py	100.00% <100.00%> (ø)
fastapi_mvc/commands/__init__.py	100.00% <100.00%> (ø)
fastapi_mvc/generators/__init__.py	100.00% <100.00%> (ø)
fastapi_mvc/generators/controller/__init__.py	100.00% <100.00%> (ø)
fastapi_mvc/generators/generator/__init__.py	100.00% <100.00%> (ø)
fastapi_mvc/generators/project/__init__.py	100.00% <100.00%> (ø)
...okiecutter.package_name}}/app/controllers/ready.py	100.00% <100.00%> (ø)
...}/{{cookiecutter.package_name}}/config/__init__.py	100.00% <100.00%> (ø)
fastapi_mvc/parsers/__init__.py	100.00% <100.00%> (ø)
... and 2 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more