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/07-external-proto-same-pkg/README.md

@@ -0,0 +1,77 @@
+# Example 07: External Proto — Same `go_package`
+
+This example demonstrates importing proto files from an external directory where both files share the **same** `go_package`.
+
+## Proto Definition
+
+Both `service.proto` and `ext.proto` use the same `go_package`:
+
+```protobuf
+option go_package = "example.com/demo/pb";
+```
+
+Source layout:
+
+```
+07-external-proto-same-pkg/
+├── ext_protos
+│   └── ext.proto        # External proto (go_package = "example.com/demo/pb")
+├── service.proto        # Service definition (go_package = "example.com/demo/pb")
+├── README.md
+└── README-cn.md
+```
+
+- `ext.proto` lives in a separate directory (`ext_protos/`), but has the same `go_package` as `service.proto`.
+- `service.proto` imports `ext.proto` and uses `ext.ExtReq` / `ext.ExtReply` as RPC types.
+
+## 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 (note `-I ./ext_protos`):
+
+```bash
+goctl rpc protoc service.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 . -I ./ext_protos
+```
+
+Generated directory structure:
+
+```
+output/
+├── etc
+│   └── svc.yaml
+├── go.mod
+├── internal
+│   ├── config
+│   │   └── config.go
+│   ├── logic
+│   │   └── querylogic.go
+│   ├── server
+│   │   └── queryserviceserver.go
+│   └── svc
+│       └── servicecontext.go
+├── pb
+│   ├── ext.pb.go
+│   ├── service.pb.go
+│   └── service_grpc.pb.go
+├── queryservice
+│   └── queryservice.go
+└── svc.go
+```
+
+## Key Points
+
+- `ext.proto` lives in a separate directory (`ext_protos/`), but has the same `go_package` as `service.proto`.
+- Use `-I ./ext_protos` to add the external directory to the proto search path.
+- When the external proto has the **same** `go_package`, all types merge into one Go package — no cross-package imports are needed.