feature/featuretags: move list of omit-able features to a Go package

Updates #12614

Change-Id: I4012c33095c6a7ccf80ad36dbab5cedbae5b3d47
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/omitsize/omitsize.go

@@ -13,17 +13,16 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
-	"regexp"
 	"slices"
 	"strconv"
 	"strings"
 
-	"tailscale.com/util/must"
+	"tailscale.com/feature/featuretags"
 )
 
 var (
 	cacheDir = flag.String("cachedir", "", "if non-empty, use this directory to store cached size results to speed up subsequent runs. The tool does not consider the git status when deciding whether to use the cache. It's on you to nuke it between runs if the tree changed.")
-	features = flag.String("features", "", "comma-separated list of features to consider, with or without the ts_omit_ prefix (default: all detected in build_dist.sh)")
+	features = flag.String("features", "", "comma-separated list of features to consider, with or without the ts_omit_ prefix")
 )
 
 func main() {
@@ -31,9 +30,9 @@ func main() {
 
 	var all []string
 	if *features == "" {
-		sh := must.Get(os.ReadFile("build_dist.sh"))
-		omitRx := regexp.MustCompile(`\b(ts_omit_\w+)\b`)
-		all = omitRx.FindAllString(string(sh), -1)
+		for k := range featuretags.Features {
+			all = append(all, "ts_omit_"+k)
+		}
 	} else {
 		for v := range strings.SplitSeq(*features, ",") {
 			if !strings.HasPrefix(v, "ts_omit_") {