GH-687: Add src/main/proto and src/test/proto as directory roots for migration purposes

[ascopes]

Closes GH-687.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.92%. Comparing base (a333e2e) to head (a972f1f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #692      +/-   ##
==========================================
+ Coverage   91.91%   91.92%   +0.01%     
==========================================
  Files          56       56              
  Lines        1667     1669       +2     
  Branches      105      105              
==========================================
+ Hits         1532     1534       +2     
  Misses         99       99              
  Partials       36       36              

Files with missing lines	Coverage Δ
...protobufmavenplugin/mojo/AbstractGenerateMojo.java	100.00% <ø> (ø)
...pes/protobufmavenplugin/mojo/MainGenerateMojo.java	100.00% <100.00%> (ø)
...pes/protobufmavenplugin/mojo/TestGenerateMojo.java	100.00% <100.00%> (ø)

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