helper/schema: emit warning when using data source resource shim

For backward compatibility we will continue to support using the data
sources that were formerly logical resources as resources for the moment,
but we want to warn the user about it since this support is likely to
be removed in future.

This is done by adding a new "deprecation message" feature to
schema.Resource, but for the moment this is done as an internal feature
(not usable directly by plugins) so that we can collect additional
use-cases and design a more general interface before creating a
compatibility constraint.

provider.go

@@ -10,6 +10,7 @@ func Provider() terraform.ResourceProvider {
 	return &schema.Provider{
 		ResourcesMap: map[string]*schema.Resource{
 			"terraform_remote_state": schema.DataSourceResourceShim(
+				"terraform_remote_state",
 				dataSourceRemoteState(),
 			),
 		},