Fix plugin generated path error #97
                
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protoc-gen-python_grpc plugin generating file's path should handle it like protobuf.
https://github.com/protocolbuffers/protobuf/blob/4e93585e8bb234efeacb7737b8d080968c5ab91e/src/google/protobuf/compiler/python/python_generator.cc#L76-L91
it handles the trailing ".protodevel" filename, replaces "-" with "_" and replaces "/" with "." in file path as a module name.
https://github.com/protocolbuffers/protobuf/blob/4e93585e8bb234efeacb7737b8d080968c5ab91e/src/google/protobuf/compiler/python/python_generator.cc#L333-L336
and replaces "." with "/" in module name as a generated file path. it removes the character "." or "-" in the directory name of the file path to create the right path of the module.