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/ioutil/append-file_nix.go

@@ -29,7 +29,7 @@ import (
 func AppendFile(dst string, src string, osync bool) error {
 	flags := os.O_WRONLY | os.O_APPEND | os.O_CREATE
 	if osync {
-		flags = flags | os.O_SYNC
+		flags |= os.O_SYNC
 	}
 	appendFile, err := os.OpenFile(dst, flags, 0666)
 	if err != nil {