Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

config/testconfig/testconfig.go

@@ -0,0 +1,84 @@
+// Copyright 2023 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.
+
+// This package should only be used for testing.
+package testconfig
+
+import (
+	_ "unsafe"
+
+	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/config/allconfig"
+	"github.com/gohugoio/hugo/deps"
+	"github.com/gohugoio/hugo/hugofs"
+	toml "github.com/pelletier/go-toml/v2"
+	"github.com/spf13/afero"
+)
+
+func GetTestConfigs(fs afero.Fs, cfg config.Provider) *allconfig.Configs {
+	if fs == nil {
+		fs = afero.NewMemMapFs()
+	}
+	if cfg == nil {
+		cfg = config.New()
+	}
+	// Make sure that the workingDir exists.
+	workingDir := cfg.GetString("workingDir")
+	if workingDir != "" {
+		if err := fs.MkdirAll(workingDir, 0777); err != nil {
+			panic(err)
+		}
+	}
+
+	configs, err := allconfig.LoadConfig(allconfig.ConfigSourceDescriptor{Fs: fs, Flags: cfg})
+	if err != nil {
+		panic(err)
+	}
+	return configs
+
+}
+
+func GetTestConfig(fs afero.Fs, cfg config.Provider) config.AllProvider {
+	return GetTestConfigs(fs, cfg).GetFirstLanguageConfig()
+}
+
+func GetTestDeps(fs afero.Fs, cfg config.Provider, beforeInit ...func(*deps.Deps)) *deps.Deps {
+	if fs == nil {
+		fs = afero.NewMemMapFs()
+	}
+	conf := GetTestConfig(fs, cfg)
+	d := &deps.Deps{
+		Conf: conf,
+		Fs:   hugofs.NewFrom(fs, conf.BaseConfig()),
+	}
+	for _, f := range beforeInit {
+		f(d)
+	}
+	if err := d.Init(); err != nil {
+		panic(err)
+	}
+	return d
+}
+
+func GetTestConfigSectionFromStruct(section string, v any) config.AllProvider {
+	data, err := toml.Marshal(v)
+	if err != nil {
+		panic(err)
+	}
+	p := maps.Params{
+		section: config.FromTOMLConfigString(string(data)).Get(""),
+	}
+	cfg := config.NewFrom(p)
+	return GetTestConfig(nil, cfg)
+}