all: use set.Set consistently instead of map[T]struct{}

I didn't clean up the more idiomatic map[T]bool with true values, at
least yet.  I just converted the relatively awkward struct{}-valued
maps.

Updates #cleanup

Change-Id: I758abebd2bb1f64bc7a9d0f25c32298f4679c14f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/set/set.go

@@ -10,6 +10,9 @@ type Set[T comparable] map[T]struct{}
 // Add adds e to the set.
 func (s Set[T]) Add(e T) { s[e] = struct{}{} }
 
+// Delete removes e from the set.
+func (s Set[T]) Delete(e T) { delete(s, e) }
+
 // Contains reports whether s contains e.
 func (s Set[T]) Contains(e T) bool {
 	_, ok := s[e]