Rewrite verify_readmes.sh script in Go (#6598)

- [x] Rewrite `verify_readmes.sh` as the package
`internal/tools/verifyreadmes`
- [x] Tweak `Makefile` to call the new script instead of the old one
- [x] Remove `verify_readmes.sh`   

Closes #6527

---------

Signed-off-by: Eliott Bouhana <eliott.bouhana@datadoghq.com>
Co-authored-by: Damien Mathieu <42@dmathieu.com>
Co-authored-by: Robert Pająk <pellared@hotmail.com>

internal/tools/verifyreadmes/main.go

@@ -0,0 +1,80 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// Package verifyreadmes is used to verify that all go modules in the repository
+// have a README.md file.
+package main
+
+import (
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// excludedDirs is a list of directories to exclude from the README check if the full path contains any of these strings.
+var excludedDirs = []string{
+	"internal",
+	"test",
+	"example",
+	"/.",
+}
+
+const readmeFilename = "README.md"
+
+// verifyReadme is a [os.WalkFunc] that checks if a README.md exists in the same directory as the go.mod file.
+func verifyReadme(path string, info os.FileInfo, err error) error {
+	if err != nil {
+		return err
+	}
+
+	if !info.Mode().IsRegular() || info.Name() != "go.mod" {
+		return nil
+
+	}
+
+	for _, dir := range excludedDirs {
+		if strings.Contains(path, dir) {
+			return nil
+		}
+	}
+
+
+	// Check that a README.md exists in the same directory as the go.mod file.
+	readme := filepath.Join(filepath.Dir(path), readmeFilename)
+	_, err = os.Stat(readme)
+	if os.IsNotExist(err) {
+		err = fmt.Errorf("couldn't find %s for %q", readmeFilename, filepath.Dir(path))
+	}
+
+	return err
+}
+
+func main() {
+	root, err := os.Getwd()
+	if len(os.Args) == 2 {
+		root, err = filepath.Abs(os.Args[1])
+	}
+
+	if err != nil {
+		fmt.Println("Error: ", err)
+		os.Exit(1)
+	}
+
+	fmt.Println("Verifying READMEs in", root)
+
+	var errs []string
+	filepath.Walk(root, func(path string, info fs.FileInfo, err error) error {
+		if err := verifyReadme(path, info, err); err != nil {
+			errs = append(errs, err.Error())
+		}
+		return nil // continue walking
+	})
+
+	if len(errs) > 0 {
+		fmt.Println("Some readme files couldn't be found.")
+		fmt.Println(strings.Join(errs, "\n"))
+		os.Exit(1)
+	}
+}