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.

modules/caddytls/fileloader.go

@@ -23,15 +23,20 @@ import (
 )
 
 func init() {
-	caddy.RegisterModule(caddy.Module{
-		Name: "tls.certificates.load_files",
-		New:  func() interface{} { return FileLoader{} },
-	})
+	caddy.RegisterModule(FileLoader{})
 }
 
 // FileLoader loads certificates and their associated keys from disk.
 type FileLoader []CertKeyFilePair
 
+// CaddyModule returns the Caddy module information.
+func (FileLoader) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		Name: "tls.certificates.load_files",
+		New:  func() caddy.Module { return new(FileLoader) },
+	}
+}
+
 // CertKeyFilePair pairs certificate and key file names along with their
 // encoding format so that they can be loaded from disk.
 type CertKeyFilePair struct {