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.