Reimplement and simplify Hugo's template system

See #13541 for details.

Fixes #13545
Fixes #13515
Closes #7964
Closes #13365
Closes #12988
Closes #4891

tpl/internal/go_templates/htmltemplate/hugo_template.go

@@ -14,6 +14,8 @@
 package template
 
 import (
+	"fmt"
+
 	"github.com/gohugoio/hugo/common/types"
 	template "github.com/gohugoio/hugo/tpl/internal/go_templates/texttemplate"
 )
@@ -51,3 +53,28 @@ func indirect(a any) any {
 
 	return in
 }
+
+// CloneShallow creates a shallow copy of the template. It does not clone  or copy the nested templates.
+func (t *Template) CloneShallow() (*Template, error) {
+	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
+	if t.escapeErr != nil {
+		return nil, fmt.Errorf("html/template: cannot Clone %q after it has executed", t.Name())
+	}
+	textClone, err := t.text.Clone()
+	if err != nil {
+		return nil, err
+	}
+	ns := &nameSpace{set: make(map[string]*Template)}
+	ns.esc = makeEscaper(ns)
+	ret := &Template{
+		nil,
+		textClone,
+		textClone.Tree,
+		ns,
+	}
+	ret.set[ret.Name()] = ret
+
+	// Return the template associated with the name of this template.
+	return ret.set[ret.Name()], nil
+}

tpl/internal/go_templates/htmltemplate/template.go

@@ -267,7 +267,7 @@ func (t *Template) Clone() (*Template, error) {
 		name := x.Name()
 		src := t.set[name]
 		if src == nil || src.escapeErr != nil {
-			return nil, fmt.Errorf("html/template: cannot Clone %q after it has executed", t.Name())
+			return nil, fmt.Errorf("html/template: cannot Clone %q after it has executed, %q not found", t.Name(), name)
 		}
 		x.Tree = x.Tree.Copy()
 		ret.set[name] = &Template{

tpl/internal/go_templates/texttemplate/example_test.go

@@ -35,7 +35,7 @@ Josie
 		Name, Gift string
 		Attended   bool
 	}
-	var recipients = []Recipient{
+	recipients := []Recipient{
 		{"Aunt Mildred", "bone china tea set", true},
 		{"Uncle John", "moleskin pants", false},
 		{"Cousin Rodney", "", false},