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/main.tf

@@ -0,0 +1,101 @@
+/**
+ * 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.
+ */
+
+
+resource "google_folder" "folder" {
+  display_name = var.name
+  parent       = var.parent
+}
+
+resource "google_folder_iam_binding" "authoritative" {
+  for_each = var.iam_roles
+  folder   = google_folder.folder.name
+  role     = each.key
+  members  = lookup(var.iam_members, each.key, [])
+}
+
+resource "google_folder_organization_policy" "boolean" {
+  for_each   = var.policy_boolean
+  folder     = google_folder.folder.name
+  constraint = each.key
+
+  dynamic boolean_policy {
+    for_each = each.value == null ? [] : [each.value]
+    iterator = policy
+    content {
+      enforced = policy.value
+    }
+  }
+
+  dynamic restore_policy {
+    for_each = each.value == null ? [""] : []
+    content {
+      default = true
+    }
+  }
+}
+
+resource "google_folder_organization_policy" "list" {
+  for_each   = var.policy_list
+  folder     = google_folder.folder.name
+  constraint = each.key
+
+  dynamic list_policy {
+    for_each = each.value.status == null ? [] : [each.value]
+    iterator = policy
+    content {
+      inherit_from_parent = policy.value.inherit_from_parent
+      suggested_value     = policy.value.suggested_value
+      dynamic allow {
+        for_each = policy.value.status ? [""] : []
+        content {
+          values = (
+            try(length(policy.value.values) > 0, false)
+            ? policy.value.values
+            : null
+          )
+          all = (
+            try(length(policy.value.values) > 0, false)
+            ? null
+            : true
+          )
+        }
+      }
+      dynamic deny {
+        for_each = policy.value.status ? [] : [""]
+        content {
+          values = (
+            try(length(policy.value.values) > 0, false)
+            ? policy.value.values
+            : null
+          )
+          all = (
+            try(length(policy.value.values) > 0, false)
+            ? null
+            : true
+          )
+        }
+      }
+    }
+  }
+
+  dynamic restore_policy {
+    for_each = each.value.status == null ? [true] : []
+    content {
+      default = true
+    }
+  }
+}