[pigeon] Add support for multiple event channel classes per definition file. #162828
Description
As mentioned flutter/packages#8566 (comment)
Currently pigeon event channels methods must all be defined in a single EventChannel annotated class. This could be confusing and overly restrictive. We should consider allowing multiple definition classes.
This will require checking for name collisions manually, since the definition file has them scope limited, while the actual generated code has all event channel methods top level.