Use `declare module.exports` syntax for flow libdefs (#5086)
Use `declare module.exports` syntax for flow libdefs (#5086)

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
bookman25/jest,ide/jest,bookman25/jest,facebook/jest,ide/jest,facebook/jest,ide/jest,cpojer/jest,facebook/jest,cpojer/jest,bookman25/jest,cpojer/jest