[refactor]: Internalize loader api

This PR intends to move the loader api to
internal. Only the necessary methods which
are needed for the api have been put into
`pkg/loader.go`.

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

api/internal/loader/errors.go

@@ -0,0 +1,11 @@
+// Copyright 2022 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package loader
+
+import "sigs.k8s.io/kustomize/kyaml/errors"
+
+var (
+	ErrHTTP     = errors.Errorf("HTTP Error")
+	ErrRtNotDir = errors.Errorf("must build at directory")
+)