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/08-external-proto-diff-pkg/README.md

@@ -0,0 +1,78 @@
+# Example 08: External Proto — Different `go_package`
+
+This example demonstrates importing proto files from an external directory where the files have **different** `go_package` values, requiring cross-package imports in the generated Go code.
+
+## Proto Definition
+
+The proto files use different `go_package` values:
+
+- `service.proto`: `go_package = "example.com/demo/pb"`
+- `ext_protos/common/types.proto`: `go_package = "example.com/demo/pb/common"`
+
+Source layout:
+
+```
+08-external-proto-diff-pkg/
+├── ext_protos
+│   └── common
+│       └── types.proto    # External proto (go_package = "example.com/demo/pb/common")
+├── service.proto          # Service definition (go_package = "example.com/demo/pb")
+├── README.md
+└── README-cn.md
+```
+
+- `types.proto` has `go_package = "example.com/demo/pb/common"` — a **different** Go package.
+- `service.proto` uses `common.ExtReq` / `common.ExtReply` directly as RPC parameter 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/
+├── dataservice
+│   └── dataservice.go
+├── etc
+│   └── svc.yaml
+├── go.mod
+├── internal
+│   ├── config
+│   │   └── config.go
+│   ├── logic
+│   │   └── fetchlogic.go
+│   ├── server
+│   │   └── dataserviceserver.go
+│   └── svc
+│       └── servicecontext.go
+├── pb
+│   ├── common
+│   │   └── types.pb.go
+│   ├── service.pb.go
+│   └── service_grpc.pb.go
+└── svc.go
+```
+
+## Key Points
+
+- When the external proto has a **different** `go_package`, goctl generates cross-package Go imports automatically.
+- goctl resolves the proto package name (e.g., `common`) to the correct Go import path by parsing the imported proto's `go_package` option.
+- `service.proto` uses `common.ExtReq` / `common.ExtReply` directly as RPC parameter types.