all: Refactor to nonglobal Viper, i18n etc.

This is a final rewrite that removes all the global state in Hugo, which also enables
the use if `t.Parallel` in tests.

Updates #2701
Fixes #3016

config/configProvider.go

@@ -0,0 +1,40 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// 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.
+
+// Package helpers implements general utility functions that work with
+// and on content.  The helper functions defined here lay down the
+// foundation of how Hugo works with files and filepaths, and perform
+// string operations on content.
+package config
+
+// A cached version of the current ConfigProvider (language) and relatives. These globals
+// are unfortunate, but we still have some places that needs this that does
+// not have access to the site configuration.
+// These values will be set on initialization when rendering a new language.
+//
+// TODO(bep) Get rid of these.
+var (
+	currentConfigProvider Provider
+)
+
+// Provider provides the configuration settings for Hugo.
+type Provider interface {
+	GetString(key string) string
+	GetInt(key string) int
+	GetBool(key string) bool
+	GetStringMap(key string) map[string]interface{}
+	GetStringMapString(key string) map[string]string
+	Get(key string) interface{}
+	Set(key string, value interface{})
+	IsSet(key string) bool
+}