Add support for finding model classes in modules

[jayqi]

Closes #23

[github-actions]

🚀 Deployed on https://deploy-preview-58--nervous-visvesvaraya-4b7042.netlify.app

[codecov-commenter]

Codecov Report

Merging #58 (655a71f) into main (df06f7e) will decrease coverage by 0.4%.
The diff coverage is 95.3%.

@@           Coverage Diff           @@
##            main     #58     +/-   ##
=======================================
- Coverage   97.9%   97.5%   -0.5%     
=======================================
  Files         13      13             
  Lines        492     533     +41     
=======================================
+ Hits         482     520     +38     
- Misses        10      13      +3     

Impacted Files	Coverage Δ
erdantic/cli.py	95.9% <91.6%> (-4.1%)	⬇️
erdantic/erd.py	98.3% <96.4%> (-0.7%)	⬇️
erdantic/base.py	97.5% <100.0%> (+0.2%)	⬆️
erdantic/exceptions.py	97.4% <100.0%> (+0.1%)	⬆️