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/04-transitive-import/main.proto

@@ -0,0 +1,19 @@
+syntax = "proto3";
+
+package pingsvc;
+
+option go_package = "example.com/demo/pb";
+
+import "middleware.proto";
+
+message PingReq {
+  middleware.RequestMeta meta = 1;
+}
+
+message PingReply {
+  string pong = 1;
+}
+
+service PingService {
+  rpc Ping(PingReq) returns (PingReply);
+}