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/02-import-sibling/README.md

@@ -0,0 +1,77 @@
+# Example 02: Importing a Sibling Proto File
+
+This example demonstrates importing a proto file from the same directory.
+
+## Proto Definition
+
+Two proto files in the same directory share the same `go_package`:
+
+- `types.proto` — Defines shared message types (`User`).
+- `user.proto` — Defines the RPC service, importing `types.proto`.
+
+Both files use the same `go_package` with a full module path:
+
+```protobuf
+option go_package = "example.com/demo/pb";
+```
+
+`user.proto` imports `types.proto` via:
+
+```protobuf
+import "types.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 user.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
+│   └── usersvc.yaml
+├── go.mod
+├── internal
+│   ├── config
+│   │   └── config.go
+│   ├── logic
+│   │   ├── createuserlogic.go
+│   │   └── getuserlogic.go
+│   ├── server
+│   │   └── userserviceserver.go
+│   └── svc
+│       └── servicecontext.go
+├── pb
+│   ├── types.pb.go
+│   ├── user.pb.go
+│   └── user_grpc.pb.go
+├── userservice
+│   └── userservice.go
+└── usersvc.go
+```
+
+## Key Points
+
+- Two proto files (`user.proto` and `types.proto`) share the same `go_package = "example.com/demo/pb"`, compiled into a single Go package.
+- `user.proto` imports `types.proto` via `import "types.proto"`.
+- When multiple proto files share the same `go_package`, they compile into a single Go package.
+- Only the proto file containing `service` definitions needs to be passed to `goctl rpc protoc`.
+- The imported proto is automatically compiled by protoc and resolved by goctl.