add gocritic/ruleguard checks back again, cleanup code. (#13665)

- remove some duplicated code
- reported a bug, separately fixed in #13664
- using strings.ReplaceAll() when needed
- using filepath.ToSlash() use when needed
- remove all non-Go style comments from the codebase

Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>

internal/s3select/csv/reader_contrib_test.go

@@ -22,7 +22,6 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
-	"path/filepath"
 	"reflect"
 	"strings"
 	"testing"
@@ -89,7 +88,7 @@ type tester interface {
 }
 
 func openTestFile(t tester, file string) []byte {
-	f, err := ioutil.ReadFile(filepath.Join("testdata/testdata.zip"))
+	f, err := ioutil.ReadFile("testdata/testdata.zip")
 	if err != nil {
 		t.Fatal(err)
 	}