goose: use marker functions instead of comments

To avoid making this CL too large, I did not migrate the existing goose
comments through the repository.  This will be addressed in a subsequent
CL.

Reviewed-by: Tuo Shan <shantuo@google.com>

goose.go

@@ -0,0 +1,38 @@
+// Package goose contains directives for goose code generation.
+package goose
+
+// ProviderSet is a marker type that collects a group of providers.
+type ProviderSet struct{}
+
+// NewSet creates a new provider set that includes the providers in
+// its arguments. Each argument is either an exported function value,
+// an exported struct (zero) value, or a call to Bind.
+func NewSet(...interface{}) ProviderSet {
+	return ProviderSet{}
+}
+
+// Use is placed in the body of an injector function to declare the
+// providers to use. Its arguments are the same as NewSet. Its return
+// value is an error message that can be sent to panic.
+//
+// Example:
+//
+//	func injector(ctx context.Context) (*sql.DB, error) {
+//		panic(Use(otherpkg.Foo, myProviderFunc, goose.Bind()))
+//	}
+func Use(...interface{}) string {
+	return "implementation not generated, run goose"
+}
+
+// A Binding maps an interface to a concrete type.
+type Binding struct{}
+
+// Bind declares that a concrete type should be used to satisfy a
+// dependency on iface.
+//
+// Example:
+//
+//	var MySet = goose.NewSet(goose.Bind(MyInterface(nil), new(MyStruct)))
+func Bind(iface, to interface{}) Binding {
+	return Binding{}
+}