Refactor Caddyfile adapter and module registration

Use piles from which to draw config values.

Module values can return their name, so now we can do two-way mapping
from value to name and name to value; whereas before we could only map
name to value. This was problematic with the Caddyfile adapter since
it receives values and needs to know the name to put in the config.

admin.go

@@ -95,9 +95,9 @@ func StartAdmin(initialConfigJSON []byte) error {
 	mux.HandleFunc("/debug/pprof/trace", pprof.Trace)
 	///// END PPROF STUFF //////
 
-	for _, m := range GetModules("admin") {
-		routes := m.New().([]AdminRoute)
-		for _, route := range routes {
+	for _, m := range GetModules("admin.routers") {
+		adminrtr := m.New().(AdminRouter)
+		for _, route := range adminrtr.Routes() {
 			mux.Handle(route.Pattern, route)
 		}
 	}
@@ -146,6 +146,11 @@ func StopAdmin() error {
 	return nil
 }
 
+// AdminRouter is a type which can return routes for the admin API.
+type AdminRouter interface {
+	Routes() []AdminRoute
+}
+
 // AdminRoute represents a route for the admin endpoint.
 type AdminRoute struct {
 	http.Handler