Cleanup ParseHostOrFile (#2100)

Create plugin/pkg/transport that holds the transport related functions.
This needed to be a new pkg to prevent cyclic import errors.

This cleans up a bunch of duplicated code in core/dnsserver that also
tried to parse a transport (now all done in transport.Parse).

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/forward/lookup_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/pkg/transport"
 	"github.com/coredns/coredns/plugin/test"
 	"github.com/coredns/coredns/request"
 
@@ -19,7 +20,7 @@ func TestLookup(t *testing.T) {
 	})
 	defer s.Close()
 
-	p := NewProxy(s.Addr, DNS)
+	p := NewProxy(s.Addr, transport.DNS)
 	f := New()
 	f.SetProxy(p)
 	defer f.Close()