refactor: extract types and pkgs to new files

pkg/types/filter.go

@@ -0,0 +1,5 @@
+package types
+
+// A Filter is a prototype for a function that can be used to filter the
+// results from a call to the ListContainers() method on the Client.
+type Filter func(FilterableContainer) bool

pkg/types/filterable_container.go

@@ -0,0 +1,9 @@
+package types
+
+// A FilterableContainer is the interface which is used to filter
+// containers.
+type FilterableContainer interface {
+	Name() string
+	IsWatchtower() bool
+	Enabled() (bool, bool)
+}

pkg/types/notifier.go

@@ -0,0 +1,6 @@
+package types
+
+type Notifier interface {
+	StartNotification()
+	SendNotification()
+}