Add base Module interface and global registry

module/module.go

@@ -0,0 +1,36 @@
+// Package module contians interfaces implemented by maddy modules.
+//
+// They are moved to separate package to prevent circular dependencies.
+//
+// Each interface required by maddy for operation is provided by some object
+// called "module".  This includes authentication, storage backends, DKIM,
+// email filters, etc.  Each module may serve multiple functions. I.e. it can
+// be IMAP storage backend, SMTP upstream and authentication provider at the
+// same moment.
+//
+// Each module gets its own unique name (sqlmail for go-sqlmail, proxy for
+// proxy module, local for local delivery perhaps, etc). Each module instance
+// also gets its own (unique too) name which is used to refer to it in
+// configuration.
+package module
+
+import "github.com/mholt/caddy/caddyfile"
+
+// Module is the interface implemented by all maddy module instances.
+//
+// It defines basic methods used to identify instances.
+type Module interface {
+	// Name method reports module name.
+	//
+	// It is used to reference module in the configuration and in logs.
+	Name() string
+
+	// InstanceName method reports unique name of this module instance.
+	InstanceName() string
+
+	// Module version. Reported in logs.
+	Version() string
+}
+
+// NewModule is function that creates new instance of module with specified name.
+type NewModule func(name string, cfg map[string]caddyfile.Token) (Module, error)