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

hugolib/page__menus.go

@@ -0,0 +1,74 @@
+// 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 hugolib
+
+import (
+	"sync"
+
+	"github.com/gohugoio/hugo/navigation"
+)
+
+type pageMenus struct {
+	p *pageState
+
+	q navigation.MenyQueryProvider
+
+	pmInit sync.Once
+	pm     navigation.PageMenus
+}
+
+func (p *pageMenus) HasMenuCurrent(menuID string, me *navigation.MenuEntry) bool {
+	p.p.s.init.menus.Do()
+	p.init()
+	return p.q.HasMenuCurrent(menuID, me)
+}
+
+func (p *pageMenus) IsMenuCurrent(menuID string, inme *navigation.MenuEntry) bool {
+	p.p.s.init.menus.Do()
+	p.init()
+	return p.q.IsMenuCurrent(menuID, inme)
+}
+
+func (p *pageMenus) Menus() navigation.PageMenus {
+	// There is a reverse dependency here. initMenus will, once, build the
+	// site menus and update any relevant page.
+	p.p.s.init.menus.Do()
+
+	return p.menus()
+}
+
+func (p *pageMenus) menus() navigation.PageMenus {
+	p.init()
+	return p.pm
+
+}
+
+func (p *pageMenus) init() {
+	p.pmInit.Do(func() {
+		p.q = navigation.NewMenuQueryProvider(
+			p.p.s.Info.sectionPagesMenu,
+			p,
+			p.p.s,
+			p.p,
+		)
+
+		var err error
+		p.pm, err = navigation.PageMenusFromPage(p.p)
+		if err != nil {
+			p.p.s.Log.ERROR.Println(p.p.wrapError(err))
+		}
+
+	})
+
+}