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/README.md

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+# Example 05: Multiple Services (`--multiple`)
+
+This example demonstrates generating multiple RPC services from a single proto file.
+
+## Proto Definition
+
+Two proto files share the same `go_package`:
+
+```protobuf
+option go_package = "example.com/demo/pb";
+```
+
+- `shared.proto` — Defines shared message types (`Meta`).
+- `multi.proto` — Defines **two** services: `SearchService` and `NotifyService`.
+
+The `-m` (or `--multiple`) flag is required when a proto file contains more than one `service` block.
+
+## 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 with the `-m` flag:
+
+```bash
+goctl rpc protoc multi.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 . \
+  -m
+```
+
+Generated directory structure:
+
+```
+output/
+├── client
+│   ├── notifyservice
+│   │   └── notifyservice.go
+│   └── searchservice
+│       └── searchservice.go
+├── etc
+│   └── multisvc.yaml
+├── go.mod
+├── internal
+│   ├── config
+│   │   └── config.go
+│   ├── logic
+│   │   ├── notifyservice
+│   │   │   └── notifylogic.go
+│   │   └── searchservice
+│   │       └── searchlogic.go
+│   ├── server
+│   │   ├── notifyservice
+│   │   │   └── notifyserviceserver.go
+│   │   └── searchservice
+│   │       └── searchserviceserver.go
+│   └── svc
+│       └── servicecontext.go
+├── multisvc.go
+└── pb
+    ├── multi.pb.go
+    ├── multi_grpc.pb.go
+    └── shared.pb.go
+```
+
+## Key Points
+
+- The `-m` (or `--multiple`) flag enables multiple-service mode.
+- In multiple mode, `client/` contains per-service subdirectories; `logic/` and `server/` are also grouped by service name.
+- Both services share a single entry point (`multisvc.go`) and config.
+- Without `--multiple`, goctl only allows one `service` block per proto file.
+- All services share the same `config.go` and `servicecontext.go`.