Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

tpl/template.go

@@ -1,4 +1,4 @@
-// Copyright 2018 The Hugo Authors. All rights reserved.
+// Copyright 2019 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.
@@ -21,6 +21,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/output"
+
 	"github.com/gohugoio/hugo/common/herrors"
 
 	"github.com/gohugoio/hugo/hugofs"
@@ -37,7 +39,8 @@ import (
 )
 
 var (
-	_ TemplateExecutor = (*TemplateAdapter)(nil)
+	_ TemplateExecutor     = (*TemplateAdapter)(nil)
+	_ TemplateInfoProvider = (*TemplateAdapter)(nil)
 )
 
 // TemplateHandler manages the collection of templates.
@@ -53,17 +56,47 @@ type TemplateHandler interface {
 	RebuildClone()
 }
 
+// TemplateVariants describes the possible variants of a template.
+// All of these may be empty.
+type TemplateVariants struct {
+	Language     string
+	OutputFormat output.Format
+}
+
 // TemplateFinder finds templates.
 type TemplateFinder interface {
+	TemplateLookup
+	TemplateLookupVariant
+}
+
+type TemplateLookup interface {
 	Lookup(name string) (Template, bool)
 }
 
+type TemplateLookupVariant interface {
+	// TODO(bep) this currently only works for shortcodes.
+	// We may unify and expand this variant pattern to the
+	// other templates, but we need this now for the shortcodes to
+	// quickly determine if a shortcode has a template for a given
+	// output format.
+	// It returns the template, if it was found or not and if there are
+	// alternative representations (output format, language).
+	// We are currently only interested in output formats, so we should improve
+	// this for speed.
+	LookupVariant(name string, variants TemplateVariants) (Template, bool, bool)
+}
+
 // Template is the common interface between text/template and html/template.
 type Template interface {
 	Execute(wr io.Writer, data interface{}) error
 	Name() string
 }
 
+// TemplateInfoProvider provides some contextual information about a template.
+type TemplateInfoProvider interface {
+	TemplateInfo() Info
+}
+
 // TemplateParser is used to parse ad-hoc templates, e.g. in the Resource chain.
 type TemplateParser interface {
 	Parse(name, tpl string) (Template, error)
@@ -92,6 +125,8 @@ type TemplateAdapter struct {
 	Template
 	Metrics metrics.Provider
 
+	Info Info
+
 	// The filesystem where the templates are stored.
 	Fs afero.Fs
 
@@ -133,6 +168,10 @@ func (t *TemplateAdapter) Execute(w io.Writer, data interface{}) (execErr error)
 	return
 }
 
+func (t *TemplateAdapter) TemplateInfo() Info {
+	return t.Info
+}
+
 // The identifiers may be truncated in the log, e.g.
 // "executing "main" at <$scaled.SRelPermalin...>: can't evaluate field SRelPermalink in type *resource.Image"
 var identifiersRe = regexp.MustCompile("at \\<(.*?)(\\.{3})?\\>:")