Remove conflicting setuptools instructions

[MattToast]

This PR fixes some conflicting directives in the SmartSim packaging instructions:

• setup.py manually listed an incomplete list of packages to include in SmartSim while the setup.cfg was using find_packages. This PR default to setup.cfg with a slightly refined include directive.
• setup.py manually listed package_data directives while the setup.cfg set include_package_data=True. This PR defaults to setup.cfg.
• In order to exclude the __pycache__ created from SmartSim modules imported during the setup.py script, __pycache__ and related files were explicitly ignored in the MANIFEST.in.
• In order to ensure the the smartsim._core.launcher.local package was found by find_packages without reverting to find_namespace_packages an __init__.py module was added to the directory.

[codecov]

Codecov Report

Merging #435 (6bf358d) into develop (d8fba1b) will decrease coverage by 0.11%.
Report is 1 commits behind head on develop.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #435      +/-   ##
===========================================
- Coverage    90.38%   90.28%   -0.11%     
===========================================
  Files           60       60              
  Lines         3839     3859      +20     
===========================================
+ Hits          3470     3484      +14     
- Misses         369      375       +6     

Files	Coverage Δ
smartsim/_core/launcher/local/local.py	95.45% <100.00%> (-0.47%)	⬇️

... and 12 files with indirect coverage changes