fix(codegen): Use wellknown types from prost-types (#49)

This change introduces the ability to let prost-build
provide the fully qualifed type path for types that
start with `google.protobuf`.

tonic-build/tests/protos/empty.proto

@@ -1,7 +0,0 @@
-syntax = "proto3";
-package empty;
-import "google/protobuf/empty.proto";
-
-service Admin {
-  rpc EmptyCall(google.protobuf.Empty) returns (google.protobuf.Empty);
-}

tonic-build/tests/protos/wellknown.proto

@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+package wellknown;
+
+import "google/protobuf/empty.proto";
+import "google/protobuf/wrappers.proto";
+import "google/protobuf/any.proto";
+
+service Admin {
+  rpc EmptyCall(google.protobuf.Empty) returns (google.protobuf.Empty);
+  rpc StringCall(google.protobuf.StringValue) returns (google.protobuf.Empty);
+  rpc AnyCall(google.protobuf.Any) returns (google.protobuf.Empty);
+}

tonic-build/tests/wellknown.rs

@@ -1,9 +1,9 @@
 #[test]
-fn empty() {
+fn wellknown() {
     let tmp = std::env::temp_dir();
     tonic_build::configure()
         .out_dir(tmp)
         .format(false)
-        .compile(&["tests/protos/empty.proto"], &["tests/protos"])
+        .compile(&["tests/protos/wellknown.proto"], &["tests/protos"])
         .unwrap();
 }