Refactor authorization challenges to its own package

Split challenges into its own package. Avoids possible
import cycle with challenges from client.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

registry/client/auth/challenge/addr.go

@@ -0,0 +1,27 @@
+package challenge
+
+import (
+	"net/url"
+	"strings"
+)
+
+// FROM: https://golang.org/src/net/http/http.go
+// Given a string of the form "host", "host:port", or "[ipv6::address]:port",
+// return true if the string includes a port.
+func hasPort(s string) bool { return strings.LastIndex(s, ":") > strings.LastIndex(s, "]") }
+
+// FROM: http://golang.org/src/net/http/transport.go
+var portMap = map[string]string{
+	"http":  "80",
+	"https": "443",
+}
+
+// canonicalAddr returns url.Host but always with a ":port" suffix
+// FROM: http://golang.org/src/net/http/transport.go
+func canonicalAddr(url *url.URL) string {
+	addr := url.Host
+	if !hasPort(addr) {
+		return addr + ":" + portMap[url.Scheme]
+	}
+	return addr
+}