go 的 protoc 插件调用逻辑

要让protoc使用插件，需要做下面事情：

Place the plugin binary somewhere in the PATH and give it the name "protoc-gen-NAME" (replacing "NAME" with the name of your plugin).

If you then invoke protoc with the parameter –NAME_out=OUT_DIR (again, replace "NAME" with your plugin's name), protoc will invoke your plugin to generate the output, which will be placed in OUT_DIR.

Place the plugin binary anywhere, with any name, and pass the –plugin parameter to protoc to direct it to your plugin like so:

protoc --plugin=protoc-gen-NAME=path/to/mybinary --NAME_out=OUT_DIR

On Windows, make sure to include the .exe suffix:

protoc --plugin=protoc-gen-NAME=path/to/mybinary.exe --NAME_out=OUT_DIR

参考： https://developers.google.com/protocol-buffers/docs/reference/cpp/google.protobuf.compiler.plugin

产生go的 grpc 代码的插件命令为：

protoc --go_out=plugins=grpc,import_path=mypackage:. *.proto

这里通过 --NAME_out ，就能知道是需要找 protoc-gen-NAME 插件， 即 protoc-gen-go 插件。
而 --go_out=plugins=grpc 则会在 https://github.com/golang/protobuf 这里找到
https://github.com/golang/protobuf/blob/master/protoc-gen-go/grpc/grpc.go 做为插件。