Wrong fully qualified message names for imported messages

[penguin138]

When generating *_grpc.py from *.proto names declared in imported protos have wrong fully qualified names. For example if I have the following proto named A.proto:

syntax = "proto3";

import "B.proto";

service ServiceA {
  rpc MethodA(InputEntityB) returns (OutputEntityB);
}

And proto named B.proto:

syntax = "proto3";

package B_package;

message InputEntityB {
     ....
}

message OutputEntityB {
     ....
}

Then in generated A_grpc.py MethodA will be defined as follows:

    service_obj.add_method(
            "MethodA", self.MethodA,
            RPCSignature(
                Cardinality.UNARY_UNARY, A.B_package.InputEntityB, A.B_package.OutputEntityB))

However, MethodA should be defined like this:

    service_obj.add_method(
            "MethodA", self.MethodA,
            RPCSignature(
                Cardinality.UNARY_UNARY, B.InputEntityB, B.OutputEntityB))

As you can see, generation code does not take into account the fact that messages used when defining services can be imported from other protos. So, I've put together a fix that solves this problem.