Move go-licenses to generate and separate generate into a frontend and backend component (#21061)

The `go-licenses` make task introduced in #21034 is being run on make vendor
and occasionally causes an empty go-licenses file if the vendors need to
change. This should be moved to the generate task as it is a generated file.

Now because of this change we also need to split generation into two separate 
steps:

1. `generate-backend`
2. `generate-frontend`

In the future it would probably be useful to make `generate-swagger` part of `generate-frontend` but it's not tolerated with our .drone.yml

Ref #21034

Signed-off-by: Andrew Thornton <art27@cantab.net>

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: delvh <dev.lh@web.de>

modules/charset/ambiguous/generate.go

@@ -110,6 +110,17 @@ func runTemplate(t *template.Template, filename string, data interface{}) error
 		verbosef("Bad source:\n%s", buf.String())
 		return fmt.Errorf("unable to format source: %w", err)
 	}
+
+	old, err := os.ReadFile(filename)
+	if err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("failed to read old file %s because %w", filename, err)
+	} else if err == nil {
+		if bytes.Equal(bs, old) {
+			// files are the same don't rewrite it.
+			return nil
+		}
+	}
+
 	file, err := os.Create(filename)
 	if err != nil {
 		return fmt.Errorf("failed to create file %s because %w", filename, err)