Revert "Remove unused container type"

This reverts commit 8aa68c0.

module.go

@@ -26,6 +26,16 @@ import (
 	"go.uber.org/dig"
 )
 
+// A container represents a set of constructors to provide
+// dependencies, and a set of functions to invoke once all the
+// dependencies have been initialized.
+//
+// This definition corresponds to the dig.Container and dig.Scope.
+type container interface {
+	Invoke(interface{}, ...dig.InvokeOption) error
+	Provide(interface{}, ...dig.ProvideOption) error
+}
+
 // Module is a named group of zero or more fx.Options. A Module is a
 // dependency graph with limited scope, and can be used for scoping
 // graph modifications (not implemented yet).