Extract several packages to form a public API

framework/module/auth.go

@@ -0,0 +1,26 @@
+package module
+
+import "errors"
+
+var (
+	// ErrUnknownCredentials should be returned by auth. provider if supplied
+	// credentials are valid for it but are not recognized (e.g. not found in
+	// used DB).
+	ErrUnknownCredentials = errors.New("unknown credentials")
+)
+
+// PlainAuth is the interface implemented by modules providing authentication using
+// username:password pairs.
+type PlainAuth interface {
+	AuthPlain(username, password string) error
+}
+
+// PlainUserDB is a local credentials store that can be managed using maddyctl
+// utility.
+type PlainUserDB interface {
+	PlainAuth
+	ListUsers() ([]string, error)
+	CreateUser(username, password string) error
+	SetUserPassword(username, password string) error
+	DeleteUser(username string) error
+}