[Refactor] Internalize konfig constants

This PR is an effort towards internalizing public APIs.
It moves some of the builtinconstants to internal/

Signed-off-by: Varsha Prasad Narsing <varshaprasad96@gmail.com>

api/internal/konfig/builtinpluginconsts/templatelabels.go

@@ -0,0 +1,8 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package builtinpluginconsts
+
+const templateLabelFieldSpecs = `
+templateLabels:
+` + metadataLabelsFieldSpecs