Move update flag to internal/source package

So it can be used by other packages.

golden/golden.go

@@ -18,13 +18,11 @@ import (
 	"gotest.tools/v3/assert"
 	"gotest.tools/v3/assert/cmp"
 	"gotest.tools/v3/internal/format"
+	"gotest.tools/v3/internal/source"
 )
 
-var flagUpdate bool
-
 func init() {
-	flag.BoolVar(&flagUpdate, "update", false, "update golden files")
-	flag.BoolVar(&flagUpdate, "test.update-golden", false, "deprecated flag")
+	flag.BoolVar(&source.Update, "test.update-golden", false, "deprecated flag")
 }
 
 type helperT interface {
@@ -46,7 +44,7 @@ var NormalizeCRLFToLF = os.Getenv("GOTESTTOOLS_GOLDEN_NormalizeCRLFToLF") != "fa
 
 // FlagUpdate returns true when the -update flag has been set.
 func FlagUpdate() bool {
-	return flagUpdate
+	return source.Update
 }
 
 // Open opens the file in ./testdata
@@ -180,7 +178,7 @@ func compare(actual []byte, filename string) (cmp.Result, []byte) {
 }
 
 func update(filename string, actual []byte) error {
-	if !flagUpdate {
+	if !source.Update {
 		return nil
 	}
 	if dir := filepath.Dir(Path(filename)); dir != "." {

golden/golden_test.go

@@ -9,6 +9,7 @@ import (
 	"gotest.tools/v3/assert"
 	"gotest.tools/v3/assert/cmp"
 	"gotest.tools/v3/fs"
+	"gotest.tools/v3/internal/source"
 )
 
 type fakeT struct {
@@ -190,10 +191,10 @@ func TestGoldenAssertBytes(t *testing.T) {
 }
 
 func setUpdateFlag(t *testing.T) func() {
-	orig := flagUpdate
-	flagUpdate = true
+	orig := source.Update
+	source.Update = true
 	undo := func() {
-		flagUpdate = orig
+		source.Update = orig
 	}
 	t.Cleanup(undo)
 	return undo

internal/source/update.go

@@ -0,0 +1,11 @@
+package source
+
+import "flag"
+
+// Update is set by the -update flag. It indicates the user running the tests
+// would like to update any golden values.
+var Update bool
+
+func init() {
+	flag.BoolVar(&Update, "update", false, "update golden files")
+}