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 mod.go file. This
file acts as a placeholder, enabling tools like golangci-lint and
go test to function correctly.

---

Discovered when working on #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 <[email protected]>

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/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/mod.go

@@ -0,0 +1,9 @@
+package mod
+
+// 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 code directly.
+
+// This would break scripts for unit testing and coverage calculation. However,
+// to ensure tools like golangci-lint and go test run normally and perform go mod checks,
+// we've added this empty file.

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