gddo-server: add UsePkgGoDev field to gddoEvent

This boolean field will reflect whether the godoc.org request was
redirected to pkg.go.dev.

Change-Id: I521c98e7342f5880021bd215640adb570935cb06
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/235957
Reviewed-by: Jonathan Amsterdam <jba@google.com>