Update folders module to Terraform 0.13

With this commit the folders module (now called simply 'folder') only
creates a single google_folder resource. Support for creating multiple
folders is no longer needed since Terraform 0.13 added for_each support
to modules.

modules/folder/variables.tf

@@ -0,0 +1,58 @@
+/**
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+variable "iam_members" {
+  description = "List of IAM members keyed by role."
+  type        = map(set(string))
+  default     = null
+}
+
+variable "iam_roles" {
+  description = "List of IAM roles."
+  type        = set(string)
+  default     = null
+}
+
+variable "name" {
+  description = "Folder name."
+  type        = string
+}
+
+variable "parent" {
+  description = "Parent in folders/folder_id or organizations/org_id format."
+  type        = string
+  validation {
+    condition     = can(regex("(organizations|folders)/[0-9]+", var.parent))
+    error_message = "Parent must be of the form folders/folder_id or organizations/organization_id."
+  }
+}
+
+variable "policy_boolean" {
+  description = "Map of boolean org policies and enforcement value, set value to null for policy restore."
+  type        = map(bool)
+  default     = {}
+}
+
+variable "policy_list" {
+  description = "Map of list org policies, status is true for allow, false for deny, null for restore. Values can only be used for allow or deny."
+  type = map(object({
+    inherit_from_parent = bool
+    suggested_value     = string
+    status              = bool
+    values              = list(string)
+  }))
+  default = {}
+}