chore: __init__.py file output is now more deterministic

[software-dov]

Sort everything that is iteratively produced, e.g. message and enum
classes provided.

[codecov]

Codecov Report

Merging #806 (8218a60) into master (7c185e8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #806   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        27    +1     
  Lines         1608      1649   +41     
  Branches       328       337    +9     
=========================================
+ Hits          1608      1649   +41     

Impacted Files	Coverage Δ
gapic/utils/case.py	100.00% <ø> (ø)
gapic/utils/reserved_names.py	100.00% <ø> (ø)
gapic/generator/generator.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <100.00%> (ø)
gapic/schema/wrappers.py	100.00% <100.00%> (ø)
gapic/utils/__init__.py	100.00% <100.00%> (ø)
gapic/utils/checks.py	100.00% <100.00%> (ø)
gapic/utils/options.py	100.00% <100.00%> (ø)

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[busunkim96]

Ah I was wondering why these still seemed to move around sometimes. Thank you!

[software-dov]

I had hoped it would get better once the generation process started using 3.7, where dicts are ordered and the air smells like warm root beer. In any case, being explicit is better.