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/05-multiple-services/multi.proto

@@ -0,0 +1,33 @@
+syntax = "proto3";
+
+package multisvc;
+
+option go_package = "example.com/demo/pb";
+
+import "shared.proto";
+
+message SearchReq {
+  shared.Meta meta    = 1;
+  string      keyword = 2;
+}
+
+message SearchReply {
+  repeated string items = 1;
+}
+
+message NotifyReq {
+  shared.Meta meta    = 1;
+  string      message = 2;
+}
+
+message NotifyReply {
+  bool ok = 1;
+}
+
+service SearchService {
+  rpc Search(SearchReq) returns (SearchReply);
+}
+
+service NotifyService {
+  rpc Notify(NotifyReq) returns (NotifyReply);
+}