After trying to explain the complexities of developing with protobuf, I
have now created a command that correctly calculates the import paths
for each package and runs the protobuf command.
The Makefile has been updated accordingly, expect we now no longer use
`go generate`. A new target `protos` has been defined. We alias the two,
for the lazy. We leave `go generate` in place for cases where we will
actually use `go generate`.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
