Use declare module.exports syntax for flow libdefs

[samwgoldman]

Summary

We added this to Flow in v0.25 (about 2 years ago), but never actually
deprecated the legacy declare var exports syntax. Hoping to do that
soon, so clearing up uses that I can find.

Test plan

flow

[codecov-io]

Codecov Report

Merging #5086 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5086   +/-   ##
=======================================
  Coverage   60.65%   60.65%           
=======================================
  Files         199      199           
  Lines        6643     6643           
  Branches        4        4           
=======================================
  Hits         4029     4029           
  Misses       2614     2614

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[SimenB]

I'm confused...

never actually deprecated the legacy declare var exports syntax.

This diff adds declare var exports.

[samwgoldman]

@SimenB Oops! Updated.

[cpojer]

Thanks for updating this @samwgoldman! :)

[github-actions]

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