extend selector with EXISTS, new tests/some old tests use table-driven style; add Has to Labels interface; enforce Requirement restrictions in constructor

pkg/labels/labels.go

@@ -23,6 +23,9 @@ import (
 
 // Labels allows you to present labels independently from their storage.
 type Labels interface {
+	// Has returns whether the provided label exists.
+	Has(label string) (exists bool)
+
 	// Get returns the value for the provided label.
 	Get(label string) (value string)
 }
@@ -42,6 +45,12 @@ func (ls Set) String() string {
 	return strings.Join(selector, ",")
 }
 
+// Has returns whether the provided label exists in the map.
+func (ls Set) Has(label string) bool {
+	_, exists := ls[label]
+	return exists
+}
+
 // Get returns the value in the map for the provided label.
 func (ls Set) Get(label string) string {
 	return ls[label]