Migrate and deprecate `sets` in favor of native starlark `set()`

[justinhorvitz]

Now that starlark has a native set (https://bazel.build/rules/lib/core/set), the skylib sets module should be deprecated. To improve performance for existing use cases, it should also be migrated to the native set instead of a dict wrapped in a struct. Typical usage is transient (not retained) so memory savings would be small, but it would also improve cpu a bit (unnecessary struct field lookup on every access).