PackageInfo.g: add GitHubUsername fields

[fingolfin]

Not yet used but in the future we may use this for
notifications and more.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.85%. Comparing base (58a9b9f) to head (baa8816).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #24   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files           5        5           
  Lines         709      709           
=======================================
  Hits          708      708           
  Misses          1        1           

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