The cookie-parser libdef contains definitions that are copy-pasted from the express libdef. This is because older versions of flow-typed did not support importing types from another libdef.
The new version of Flow doesn't like this, as it's not able to conclude that the copy-pasted types are the same as the original types, even if I update them to match.
Instead, I've reimplemented the cookie-parser libdef without any copy-paste, and using imports instead.
NOTE: CI will fail on this diff. I considered the possibility of fixing Flow errors BEFORE upgrading Flow, but it wasn't possible... in some cases, the fixes to support the new version of Flow caused errors in the old version. I could have hidden these type errors with $FlowFixMe lines and then later revert those, but that seemed like too much busy work.
Depends on D9861