feat(goctl/rpc): support external proto imports with cross-package ty… (#5472)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tools/goctl/rpc/example/01-basic/greeter.proto

@@ -0,0 +1,17 @@
+syntax = "proto3";
+
+package greeter;
+
+option go_package = "example.com/demo/greeter";
+
+message HelloReq {
+  string name = 1;
+}
+
+message HelloReply {
+  string message = 1;
+}
+
+service Greeter {
+  rpc SayHello(HelloReq) returns (HelloReply);
+}