types/dnstype: delete unused func, move other one to its sole caller

The dnstype package is used by tailcfg, which tries to be light and
leafy. But it brings in dnstype. So dnstype shouldn't bring in
x/net/dns/dnsmessage.

Updates #12614

Change-Id: I043637a7ce7fed097e648001f13ca1927a781def
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/localapi/localapi.go

@@ -38,7 +38,6 @@ import (
 	"tailscale.com/net/netutil"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstime"
-	"tailscale.com/types/dnstype"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/logid"
@@ -1995,7 +1994,7 @@ func (h *Handler) serveDNSQuery(w http.ResponseWriter, r *http.Request) {
 	queryType := q.Get("type")
 	qt := dnsmessage.TypeA
 	if queryType != "" {
-		t, err := dnstype.DNSMessageTypeForString(queryType)
+		t, err := dnsMessageTypeForString(queryType)
 		if err != nil {
 			http.Error(w, err.Error(), http.StatusBadRequest)
 			return
@@ -2016,6 +2015,43 @@ func (h *Handler) serveDNSQuery(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// dnsMessageTypeForString returns the dnsmessage.Type for the given string.
+// For example, DNSMessageTypeForString("A") returns dnsmessage.TypeA.
+func dnsMessageTypeForString(s string) (t dnsmessage.Type, err error) {
+	s = strings.TrimSpace(strings.ToUpper(s))
+	switch s {
+	case "AAAA":
+		return dnsmessage.TypeAAAA, nil
+	case "ALL":
+		return dnsmessage.TypeALL, nil
+	case "A":
+		return dnsmessage.TypeA, nil
+	case "CNAME":
+		return dnsmessage.TypeCNAME, nil
+	case "HINFO":
+		return dnsmessage.TypeHINFO, nil
+	case "MINFO":
+		return dnsmessage.TypeMINFO, nil
+	case "MX":
+		return dnsmessage.TypeMX, nil
+	case "NS":
+		return dnsmessage.TypeNS, nil
+	case "OPT":
+		return dnsmessage.TypeOPT, nil
+	case "PTR":
+		return dnsmessage.TypePTR, nil
+	case "SOA":
+		return dnsmessage.TypeSOA, nil
+	case "SRV":
+		return dnsmessage.TypeSRV, nil
+	case "TXT":
+		return dnsmessage.TypeTXT, nil
+	case "WKS":
+		return dnsmessage.TypeWKS, nil
+	}
+	return 0, errors.New("unknown DNS message type: " + s)
+}
+
 // serveSuggestExitNode serves a POST endpoint for returning a suggested exit node.
 func (h *Handler) serveSuggestExitNode(w http.ResponseWriter, r *http.Request) {
 	if r.Method != httpm.GET {