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>