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

resources/page/page_outputformat.go

@@ -0,0 +1,85 @@
+// 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.
+// 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.
+
+// Package page contains the core interfaces and types for the Page resource,
+// a core component in Hugo.
+package page
+
+import (
+	"strings"
+
+	"github.com/gohugoio/hugo/media"
+	"github.com/gohugoio/hugo/output"
+)
+
+// OutputFormats holds a list of the relevant output formats for a given page.
+type OutputFormats []OutputFormat
+
+// OutputFormat links to a representation of a resource.
+type OutputFormat struct {
+	// Rel constains a value that can be used to construct a rel link.
+	// This is value is fetched from the output format definition.
+	// Note that for pages with only one output format,
+	// this method will always return "canonical".
+	// As an example, the AMP output format will, by default, return "amphtml".
+	//
+	// See:
+	// https://www.ampproject.org/docs/guides/deploy/discovery
+	//
+	// Most other output formats will have "alternate" as value for this.
+	Rel string
+
+	Format output.Format
+
+	relPermalink string
+	permalink    string
+}
+
+// Name returns this OutputFormat's name, i.e. HTML, AMP, JSON etc.
+func (o OutputFormat) Name() string {
+	return o.Format.Name
+}
+
+// MediaType returns this OutputFormat's MediaType (MIME type).
+func (o OutputFormat) MediaType() media.Type {
+	return o.Format.MediaType
+}
+
+// Permalink returns the absolute permalink to this output format.
+func (o OutputFormat) Permalink() string {
+	return o.permalink
+}
+
+// RelPermalink returns the relative permalink to this output format.
+func (o OutputFormat) RelPermalink() string {
+	return o.relPermalink
+}
+
+func NewOutputFormat(relPermalink, permalink string, isCanonical bool, f output.Format) OutputFormat {
+	rel := f.Rel
+	if isCanonical {
+		rel = "canonical"
+	}
+	return OutputFormat{Rel: rel, Format: f, relPermalink: relPermalink, permalink: permalink}
+}
+
+// Get gets a OutputFormat given its name, i.e. json, html etc.
+// It returns nil if none found.
+func (o OutputFormats) Get(name string) *OutputFormat {
+	for _, f := range o {
+		if strings.EqualFold(f.Format.Name, name) {
+			return &f
+		}
+	}
+	return nil
+}