tpl: Rework to handle both text and HTML templates

Before this commit, Hugo used `html/template` for all Go templates.

While this is a fine choice for HTML and maybe also RSS feeds, it is painful for plain text formats such as CSV, JSON etc.

This commit fixes that by using the `IsPlainText` attribute on the output format to decide what to use.

A couple of notes:

* The above requires a nonambiguous template name to type mapping. I.e. `/layouts/_default/list.json` will only work if there is only one JSON output format, `/layouts/_default/list.mytype.json` will always work.
* Ambiguous types will fall back to HTML.
* Partials inherits the text vs HTML identificator of the container template. This also means that plain text templates can only include plain text partials.
* Shortcode templates are, by definition, currently HTML templates only.

Fixes #3221

tpl/tplimpl/template_ast_transformers.go

@@ -17,6 +17,7 @@ import (
 	"errors"
 	"html/template"
 	"strings"
+	texttemplate "text/template"
 	"text/template/parse"
 )
 
@@ -35,32 +36,57 @@ var paramsPaths = [][]string{
 }
 
 type templateContext struct {
-	decl    decl
-	templ   *template.Template
-	visited map[string]bool
+	decl     decl
+	visited  map[string]bool
+	lookupFn func(name string) *parse.Tree
 }
 
-func (c templateContext) getIfNotVisited(name string) *template.Template {
+func (c templateContext) getIfNotVisited(name string) *parse.Tree {
 	if c.visited[name] {
 		return nil
 	}
 	c.visited[name] = true
-	return c.templ.Lookup(name)
+	return c.lookupFn(name)
 }
 
-func newTemplateContext(templ *template.Template) *templateContext {
-	return &templateContext{templ: templ, decl: make(map[string]string), visited: make(map[string]bool)}
+func newTemplateContext(lookupFn func(name string) *parse.Tree) *templateContext {
+	return &templateContext{lookupFn: lookupFn, decl: make(map[string]string), visited: make(map[string]bool)}
 
 }
 
-func applyTemplateTransformers(templ *template.Template) error {
-	if templ == nil || templ.Tree == nil {
+func createParseTreeLookup(templ *template.Template) func(nn string) *parse.Tree {
+	return func(nn string) *parse.Tree {
+		tt := templ.Lookup(nn)
+		if tt != nil {
+			return tt.Tree
+		}
+		return nil
+	}
+}
+
+func applyTemplateTransformersToHMLTTemplate(templ *template.Template) error {
+	return applyTemplateTransformers(templ.Tree, createParseTreeLookup(templ))
+}
+
+func applyTemplateTransformersToTextTemplate(templ *texttemplate.Template) error {
+	return applyTemplateTransformers(templ.Tree,
+		func(nn string) *parse.Tree {
+			tt := templ.Lookup(nn)
+			if tt != nil {
+				return tt.Tree
+			}
+			return nil
+		})
+}
+
+func applyTemplateTransformers(templ *parse.Tree, lookupFn func(name string) *parse.Tree) error {
+	if templ == nil {
 		return errors.New("expected template, but none provided")
 	}
 
-	c := newTemplateContext(templ)
+	c := newTemplateContext(lookupFn)
 
-	c.paramsKeysToLower(templ.Tree.Root)
+	c.paramsKeysToLower(templ.Root)
 
 	return nil
 }
@@ -84,7 +110,7 @@ func (c *templateContext) paramsKeysToLower(n parse.Node) {
 	case *parse.TemplateNode:
 		subTempl := c.getIfNotVisited(x.Name)
 		if subTempl != nil {
-			c.paramsKeysToLowerForNodes(subTempl.Tree.Root)
+			c.paramsKeysToLowerForNodes(subTempl.Root)
 		}
 	case *parse.PipeNode:
 		for i, elem := range x.Decl {