Use src-based project structure

[youtux]

I will need this for the tatsu-parser branch.

[codecov[bot]]

Codecov Report

Merging #552 (d080736) into master (6efbc36) will increase coverage by 3.24%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   95.25%   98.49%   +3.24%     
==========================================
  Files          48       32      -16     
  Lines        1708      733     -975     
  Branches      209       18     -191     
==========================================
- Hits         1627      722     -905     
+ Misses         51        9      -42     
+ Partials       30        2      -28     

Impacted Files	Coverage Δ
src/pytest_bdd/__init__.py
src/pytest_bdd/cucumber_json.py
src/pytest_bdd/exceptions.py
src/pytest_bdd/feature.py
src/pytest_bdd/generation.py
src/pytest_bdd/gherkin_terminal_reporter.py
src/pytest_bdd/hooks.py
src/pytest_bdd/parser.py
src/pytest_bdd/parsers.py
src/pytest_bdd/plugin.py
... and 6 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.