Allow creating packages for master/main. (#116557)

This is to enable testing of the packaging scripts on master/main
branches.

Bug: https://github.com/flutter/flutter/issues/115489

dev/bots/prepare_package.dart

@@ -54,7 +54,9 @@ class PreparePackageException implements Exception {
 
 enum Branch {
   beta,
-  stable;
+  stable,
+  master,
+  main;
 }
 
 /// A helper class for classes that want to run a process, optionally have the