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/crypto/key_test.go

@@ -164,7 +164,7 @@ func TestDerivePartKey(t *testing.T) {
 			t.Fatalf("Test %d failed to decode expected part-key: %v", i, err)
 		}
 		partKey := key.DerivePartKey(test.PartID)
-		if !bytes.Equal(partKey[:], expectedPartKey[:]) {
+		if !bytes.Equal(partKey[:], expectedPartKey) {
 			t.Errorf("Test %d derives wrong part-key: got '%s' want: '%s'", i, hex.EncodeToString(partKey[:]), test.PartKey)
 		}
 	}