helper/schema: Add schemaMap.DeepCopy

This provides a deep copy of a schemaMap, which will be needed for the
diff customization process as ResourceDiff will be able to flag fields
as ForceNew - we don't want to affect the source schema.

helper/schema/schema.go

@@ -21,6 +21,7 @@ import (
 	"strings"
 
 	"github.com/hashicorp/terraform/terraform"
+	"github.com/mitchellh/copystructure"
 	"github.com/mitchellh/mapstructure"
 )
 
@@ -370,6 +371,16 @@ func (m schemaMap) Data(
 	}, nil
 }
 
+// DeepCopy returns a copy of this schemaMap. The copy can be safely modified
+// without affecting the original.
+func (m *schemaMap) DeepCopy() schemaMap {
+	copy, err := copystructure.Config{Lock: true}.Copy(m)
+	if err != nil {
+		panic(err)
+	}
+	return copy.(schemaMap)
+}
+
 // Diff returns the diff for a resource given the schema map,
 // state, and configuration.
 func (m schemaMap) Diff(