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

hugolib/hugo_smoke_test.go

@@ -1,4 +1,4 @@
-// Copyright 2019 The Hugo Authors. All rights reserved.
+// 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.
@@ -24,23 +24,27 @@ import (
 
 // The most basic build test.
 func TestHello(t *testing.T) {
-	t.Parallel()
-	b := newTestSitesBuilder(t)
-	b.WithConfigFile("toml", `
+	files := `
+-- hugo.toml --
+title = "Hello"
 baseURL="https://example.org"
 disableKinds = ["term", "taxonomy", "section", "page"]
-`)
-	b.WithContent("p1", `
+-- content/p1.md --
 ---
 title: Page
 ---
+-- layouts/index.html --
+{{ .Title }}
+`
 
-`)
-	b.WithTemplates("index.html", `Site: {{ .Site.Language.Lang | upper }}`)
+	b := NewIntegrationTestBuilder(
+		IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+		},
+	).Build()
 
-	b.Build(BuildCfg{})
-
-	b.AssertFileContent("public/index.html", `Site: EN`)
+	b.AssertFileContent("public/index.html", `Hello`)
 }
 
 func TestSmoke(t *testing.T) {