Missing betterproto_lib_google_protobuf import if only used as rpc call parameter.

I'm using 2.0.0b5.

The following file results in broken python:

```proto
syntax = "proto3";

package experiment;

import "google/protobuf/empty.proto";

message Reply {
bool happy = 1;
}

service Testcase {
rpc call(google.protobuf.Empty) returns (Reply);
}
```

Specifically, the import `import betterproto.lib.google.protobuf as betterproto_lib_google_protobuf` is missing from the output file.

When any `google.protobuf` imported field is used anywhere else, it does work correctly, for example, the following files all work:

```proto
syntax = "proto3";

package experiment;

import "google/protobuf/empty.proto";

message Reply {
bool happy = 1;
}

service Testcase {
rpc call(Reply) returns (google.protobuf.Empty);
}
```

```proto
syntax = "proto3";

package experiment;

import "google/protobuf/empty.proto";

message Reply {
bool happy = 1;
google.protobuf.Empty e = 2;
}

service Testcase {
rpc call(google.protobuf.Empty) returns (Reply);
}

```