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

1 files changed, 463 insertions, 0 deletions

diff --git a/resources/page/page_nop.go b/resources/page/page_nop.go
new file mode 100644
index 000000000..7afbee216
--- /dev/null
+++ b/resources/page/page_nop.go
@@ -0,0 +1,463 @@
+// 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 (
+	"html/template"
+	"os"
+	"time"
+
+	"github.com/bep/gitmap"
+	"github.com/gohugoio/hugo/navigation"
+
+	"github.com/gohugoio/hugo/common/hugo"
+	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/source"
+
+	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/langs"
+	"github.com/gohugoio/hugo/media"
+	"github.com/gohugoio/hugo/related"
+	"github.com/gohugoio/hugo/resources/resource"
+)
+
+var (
+	NopPage Page = new(nopPage)
+	NilPage *nopPage
+)
+
+// PageNop implements Page, but does nothing.
+type nopPage int
+
+func (p *nopPage) Aliases() []string {
+	return nil
+}
+
+func (p *nopPage) Sitemap() config.Sitemap {
+	return config.Sitemap{}
+}
+
+func (p *nopPage) Layout() string {
+	return ""
+}
+
+func (p *nopPage) RSSLink() template.URL {
+	return ""
+}
+
+func (p *nopPage) Author() Author {
+	return Author{}
+
+}
+func (p *nopPage) Authors() AuthorList {
+	return nil
+}
+
+func (p *nopPage) AllTranslations() Pages {
+	return nil
+}
+
+func (p *nopPage) LanguagePrefix() string {
+	return ""
+}
+
+func (p *nopPage) AlternativeOutputFormats() OutputFormats {
+	return nil
+}
+
+func (p *nopPage) BaseFileName() string {
+	return ""
+}
+
+func (p *nopPage) BundleType() string {
+	return ""
+}
+
+func (p *nopPage) Content() (interface{}, error) {
+	return "", nil
+}
+
+func (p *nopPage) ContentBaseName() string {
+	return ""
+}
+
+func (p *nopPage) CurrentSection() Page {
+	return nil
+}
+
+func (p *nopPage) Data() interface{} {
+	return nil
+}
+
+func (p *nopPage) Date() (t time.Time) {
+	return
+}
+
+func (p *nopPage) Description() string {
+	return ""
+}
+
+func (p *nopPage) RefFrom(argsm map[string]interface{}, source interface{}) (string, error) {
+	return "", nil
+}
+func (p *nopPage) RelRefFrom(argsm map[string]interface{}, source interface{}) (string, error) {
+	return "", nil
+}
+
+func (p *nopPage) Dir() string {
+	return ""
+}
+
+func (p *nopPage) Draft() bool {
+	return false
+}
+
+func (p *nopPage) Eq(other interface{}) bool {
+	return p == other
+}
+
+func (p *nopPage) ExpiryDate() (t time.Time) {
+	return
+}
+
+func (p *nopPage) Ext() string {
+	return ""
+}
+
+func (p *nopPage) Extension() string {
+	return ""
+}
+
+var nilFile *source.FileInfo
+
+func (p *nopPage) File() source.File {
+	return nilFile
+}
+
+func (p *nopPage) FileInfo() os.FileInfo {
+	return nil
+}
+
+func (p *nopPage) Filename() string {
+	return ""
+}
+
+func (p *nopPage) FirstSection() Page {
+	return nil
+}
+
+func (p *nopPage) FuzzyWordCount() int {
+	return 0
+}
+
+func (p *nopPage) GetPage(ref string) (Page, error) {
+	return nil, nil
+}
+
+func (p *nopPage) GetParam(key string) interface{} {
+	return nil
+}
+
+func (p *nopPage) GitInfo() *gitmap.GitInfo {
+	return nil
+}
+
+func (p *nopPage) HasMenuCurrent(menuID string, me *navigation.MenuEntry) bool {
+	return false
+}
+
+func (p *nopPage) HasShortcode(name string) bool {
+	return false
+}
+
+func (p *nopPage) Hugo() (h hugo.Info) {
+	return
+}
+
+func (p *nopPage) InSection(other interface{}) (bool, error) {
+	return false, nil
+}
+
+func (p *nopPage) IsAncestor(other interface{}) (bool, error) {
+	return false, nil
+}
+
+func (p *nopPage) IsDescendant(other interface{}) (bool, error) {
+	return false, nil
+}
+
+func (p *nopPage) IsDraft() bool {
+	return false
+}
+
+func (p *nopPage) IsHome() bool {
+	return false
+}
+
+func (p *nopPage) IsMenuCurrent(menuID string, inme *navigation.MenuEntry) bool {
+	return false
+}
+
+func (p *nopPage) IsNode() bool {
+	return false
+}
+
+func (p *nopPage) IsPage() bool {
+	return false
+}
+
+func (p *nopPage) IsSection() bool {
+	return false
+}
+
+func (p *nopPage) IsTranslated() bool {
+	return false
+}
+
+func (p *nopPage) Keywords() []string {
+	return nil
+}
+
+func (p *nopPage) Kind() string {
+	return ""
+}
+
+func (p *nopPage) Lang() string {
+	return ""
+}
+
+func (p *nopPage) Language() *langs.Language {
+	return nil
+}
+
+func (p *nopPage) Lastmod() (t time.Time) {
+	return
+}
+
+func (p *nopPage) Len() int {
+	return 0
+}
+
+func (p *nopPage) LinkTitle() string {
+	return ""
+}
+
+func (p *nopPage) LogicalName() string {
+	return ""
+}
+
+func (p *nopPage) MediaType() (m media.Type) {
+	return
+}
+
+func (p *nopPage) Menus() (m navigation.PageMenus) {
+	return
+}
+
+func (p *nopPage) Name() string {
+	return ""
+}
+
+func (p *nopPage) Next() Page {
+	return nil
+}
+
+func (p *nopPage) OutputFormats() OutputFormats {
+	return nil
+}
+
+func (p *nopPage) Pages() Pages {
+	return nil
+}
+
+func (p *nopPage) Paginate(seq interface{}, options ...interface{}) (*Pager, error) {
+	return nil, nil
+}
+
+func (p *nopPage) Paginator(options ...interface{}) (*Pager, error) {
+	return nil, nil
+}
+
+func (p *nopPage) Param(key interface{}) (interface{}, error) {
+	return nil, nil
+}
+
+func (p *nopPage) Params() map[string]interface{} {
+	return nil
+}
+
+func (p *nopPage) Parent() Page {
+	return nil
+}
+
+func (p *nopPage) Path() string {
+	return ""
+}
+
+func (p *nopPage) Permalink() string {
+	return ""
+}
+
+func (p *nopPage) Plain() string {
+	return ""
+}
+
+func (p *nopPage) PlainWords() []string {
+	return nil
+}
+
+func (p *nopPage) Prev() Page {
+	return nil
+}
+
+func (p *nopPage) PublishDate() (t time.Time) {
+	return
+}
+
+func (p *nopPage) PrevInSection() Page {
+	return nil
+}
+func (p *nopPage) NextInSection() Page {
+	return nil
+}
+
+func (p *nopPage) PrevPage() Page {
+	return nil
+}
+
+func (p *nopPage) NextPage() Page {
+	return nil
+}
+
+func (p *nopPage) RawContent() string {
+	return ""
+}
+
+func (p *nopPage) ReadingTime() int {
+	return 0
+}
+
+func (p *nopPage) Ref(argsm map[string]interface{}) (string, error) {
+	return "", nil
+}
+
+func (p *nopPage) RelPermalink() string {
+	return ""
+}
+
+func (p *nopPage) RelRef(argsm map[string]interface{}) (string, error) {
+	return "", nil
+}
+
+func (p *nopPage) Render(layout ...string) template.HTML {
+	return ""
+}
+
+func (p *nopPage) ResourceType() string {
+	return ""
+}
+
+func (p *nopPage) Resources() resource.Resources {
+	return nil
+}
+
+func (p *nopPage) Scratch() *maps.Scratch {
+	return nil
+}
+
+func (p *nopPage) RelatedKeywords(cfg related.IndexConfig) ([]related.Keyword, error) {
+	return nil, nil
+}
+
+func (p *nopPage) Section() string {
+	return ""
+}
+
+func (p *nopPage) Sections() Pages {
+	return nil
+}
+
+func (p *nopPage) SectionsEntries() []string {
+	return nil
+}
+
+func (p *nopPage) SectionsPath() string {
+	return ""
+}
+
+func (p *nopPage) Site() Site {
+	return nil
+}
+
+func (p *nopPage) Sites() Sites {
+	return nil
+}
+
+func (p *nopPage) Slug() string {
+	return ""
+}
+
+func (p *nopPage) String() string {
+	return "nopPage"
+}
+
+func (p *nopPage) Summary() template.HTML {
+	return ""
+}
+
+func (p *nopPage) TableOfContents() template.HTML {
+	return ""
+}
+
+func (p *nopPage) Title() string {
+	return ""
+}
+
+func (p *nopPage) TranslationBaseName() string {
+	return ""
+}
+
+func (p *nopPage) TranslationKey() string {
+	return ""
+}
+
+func (p *nopPage) Translations() Pages {
+	return nil
+}
+
+func (p *nopPage) Truncated() bool {
+	return false
+}
+
+func (p *nopPage) Type() string {
+	return ""
+}
+
+func (p *nopPage) URL() string {
+	return ""
+}
+
+func (p *nopPage) UniqueID() string {
+	return ""
+}
+
+func (p *nopPage) Weight() int {
+	return 0
+}
+
+func (p *nopPage) WordCount() int {
+	return 0
+}