Split into two proto files and modify appropriate build rules for
configure / scons / meson builds.

There are private internal events (proxy) that communicate information
from rasterizer to ArchRast. ArchRast can use these events to calculate
a final answer and then emit other public events which will be saved to
file. Users will use the public proto file and not the private one.

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>