The singleflight package started life in internal/singleflight and then moved to golang.org/x/sync/singleflight, and the two have had slightly divergent development histories since. It's getting kinda messy. Their APIs have even somewhat diverged.

I just tried to re-unify them and delete internal/singleflight from std, but we can't use mod/vendor packages during bootstrap.

/cc @ianlancetaylor