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.go

@@ -7,6 +7,7 @@ package forward
 import (
 	"context"
 
+	"github.com/coredns/coredns/plugin/pkg/transport"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -81,7 +82,7 @@ func (f *Forward) Lookup(state request.Request, name string, typ uint16) (*dns.M
 func NewLookup(addr []string) *Forward {
 	f := New()
 	for i := range addr {
-		p := NewProxy(addr[i], DNS)
+		p := NewProxy(addr[i], transport.DNS)
 		f.SetProxy(p)
 	}
 	return f