Move genflags tool to cmd/ and pin to previous release

to alleviate problems caused by the circular dependency of using the
same code as a library that is potentially being generated to adhere to
a different API.

internal/build/build.go

@@ -94,7 +94,7 @@ func main() {
 		},
 		&cli.StringSliceFlag{
 			Name:  "packages",
-			Value: cli.NewStringSlice("cli", "altsrc", "internal/build", "internal/genflags"),
+			Value: cli.NewStringSlice("cli", "altsrc", "internal/build"),
 		},
 	}