Add tools/mod to module_dirs

As `tools/mod` also contains the `go.mod` file. We should add it to the
`module_dirs` variable, so that when executing `./scripts/fix.sh`, the
proper checks and fixes can be applied.

To address the issue of broken unit tests and code coverage due to the
directory's lack of Go code, we've introduced a new doc.go file. This
file acts as a placeholder, enabling tools like golangci-lint and
go test to function correctly.

---

Discovered when working on https://github.com/etcd-io/etcd/pull/18575

The directories are checked against the following:
- Command: `find . -type f -name go.mod -exec dirname {} \;`
- Output:
```
./etcdutl
.
./tools/testgrid-analysis
./tools/rw-heatmaps
./tools/mod
./etcdctl
./tests
./server
./api
./client/internal/v2
./client/v3
./client/pkg
./pkg
```

Signed-off-by: Chun-Hung Tseng <henrybear327@gmail.com>

scripts/test_lib.sh

@@ -166,7 +166,7 @@ function run_for_module {
 }
 
 function module_dirs() {
-  echo "api pkg client/pkg client/internal/v2 client/v3 server etcdutl etcdctl tests tools/rw-heatmaps tools/testgrid-analysis ."
+  echo "api pkg client/pkg client/internal/v2 client/v3 server etcdutl etcdctl tests tools/mod tools/rw-heatmaps tools/testgrid-analysis ."
 }
 
 # maybe_run [cmd...] runs given command depending on the DRY_RUN flag.

tools/mod/doc.go

@@ -0,0 +1,22 @@
+// Copyright 2024 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// As this directory implements the pattern for tracking tool dependencies as documented here:
+// https://go.dev/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module, it doesn't
+// contain any valid go source code in the directory directly. This would break scripts for
+// unit testing, golangci-lint, and coverage calculation.
+//
+// Thus, to ensure tools to run normally, we've added this empty file.
+
+package mod

tools/mod/libs.go

@@ -15,7 +15,7 @@
 //go:build libs
 
 // This file implements that pattern:
-// https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module
+// https://go.dev/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module
 // for etcd. Thanks to this file 'go mod tidy' does not removes dependencies.
 
 package libs

tools/mod/tools.go

@@ -15,7 +15,7 @@
 //go:build tools
 
 // This file implements that pattern:
-// https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module
+// https://go.dev/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module
 // for etcd. Thanks to this file 'go mod tidy' does not removes dependencies.
 
 package tools