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/README.md

@@ -0,0 +1,72 @@
+# Example 04: Transitive Imports
+
+This example demonstrates transitive proto imports, where A imports B and B imports C.
+
+## Proto Definition
+
+Three proto files form a transitive import chain, all sharing the same `go_package`:
+
+```protobuf
+option go_package = "example.com/demo/pb";
+```
+
+- `base.proto` — Layer C: defines base types (`BaseResp`).
+- `middleware.proto` — Layer B: imports `base.proto`, defines `RequestMeta`.
+- `main.proto` — Layer A: imports `middleware.proto`, defines the `PingService` (entry point).
+
+Import chain: `main.proto` → `middleware.proto` → `base.proto`
+
+## Generation Commands
+
+First, initialize the output directory with a `go.mod`:
+
+```bash
+mkdir -p output && cd output && go mod init example.com/demo && cd ..
+```
+
+Then generate the code:
+
+```bash
+goctl rpc protoc main.proto \
+  --go_out=output \
+  --go-grpc_out=output \
+  --zrpc_out=output \
+  --go_opt=module=example.com/demo \
+  --go-grpc_opt=module=example.com/demo \
+  --module=example.com/demo \
+  -I .
+```
+
+Generated directory structure:
+
+```
+output/
+├── etc
+│   └── pingsvc.yaml
+├── go.mod
+├── internal
+│   ├── config
+│   │   └── config.go
+│   ├── logic
+│   │   └── pinglogic.go
+│   ├── server
+│   │   └── pingserviceserver.go
+│   └── svc
+│       └── servicecontext.go
+├── pb
+│   ├── base.pb.go
+│   ├── main.pb.go
+│   ├── main_grpc.pb.go
+│   └── middleware.pb.go
+├── pingservice
+│   └── pingservice.go
+└── pingsvc.go
+```
+
+## Key Points
+
+- Three proto files (`base.proto` → `middleware.proto` → `main.proto`) form a transitive import chain.
+- goctl recursively resolves all transitive imports automatically.
+- All three files share the same `go_package = "example.com/demo/pb"`.
+- You only need to specify the entry proto file — goctl and protoc handle the rest.
+- Circular imports are detected and will cause an error (same as protoc behavior).