This is where the magic starts to happen. We move includes from the base
protocol header (${NAME}.h) to the parent and child headers (${NAME}Parent.h
and ${NAME}Child.h) to follow good include hygiene.
For the base protocol header, we examine the set of types used by struct and
union definitions to determine which headers we need to include.
For parent and child headers, we forward declare what we can and include
appropriate headers otherwise. For forward-declared types, we include the
appropriate headers in the parent and child source files.
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