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__output.go

@@ -0,0 +1,107 @@
+// 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 (
+	"github.com/gohugoio/hugo/output"
+	"github.com/gohugoio/hugo/resources/page"
+	"github.com/gohugoio/hugo/resources/resource"
+)
+
+func newPageOutput(
+	cp *pageContentOutput, // may be nil
+	ps *pageState,
+	pp pagePaths,
+	f output.Format,
+	render bool) *pageOutput {
+
+	var targetPathsProvider targetPathsHolder
+	var linksProvider resource.ResourceLinksProvider
+
+	ft, found := pp.targetPaths[f.Name]
+	if !found {
+		// Link to the main output format
+		ft = pp.targetPaths[pp.OutputFormats()[0].Format.Name]
+	}
+	targetPathsProvider = ft
+	linksProvider = ft
+
+	var paginatorProvider page.PaginatorProvider = page.NopPage
+	var pag *pagePaginator
+
+	if render && ps.IsNode() {
+		pag = &pagePaginator{source: ps}
+		paginatorProvider = pag
+	}
+
+	var contentProvider page.ContentProvider = page.NopPage
+	var tableOfContentsProvider page.TableOfContentsProvider = page.NopPage
+
+	if cp != nil {
+		contentProvider = cp
+		tableOfContentsProvider = cp
+	}
+
+	providers := struct {
+		page.ContentProvider
+		page.TableOfContentsProvider
+		page.PaginatorProvider
+		resource.ResourceLinksProvider
+		targetPather
+	}{
+		contentProvider,
+		tableOfContentsProvider,
+		paginatorProvider,
+		linksProvider,
+		targetPathsProvider,
+	}
+
+	po := &pageOutput{
+		f:                      f,
+		cp:                     cp,
+		pagePerOutputProviders: providers,
+		render:                 render,
+		paginator:              pag,
+	}
+
+	return po
+
+}
+
+// We create a pageOutput for every output format combination, even if this
+// particular page isn't configured to be rendered to that format.
+type pageOutput struct {
+	// Set if this page isn't configured to be rendered to this format.
+	render bool
+
+	f output.Format
+
+	// Only set if render is set.
+	// Note that this will be lazily initialized, so only used if actually
+	// used in template(s).
+	paginator *pagePaginator
+
+	// This interface provides the functionality that is specific for this
+	// output format.
+	pagePerOutputProviders
+
+	// This may be nil.
+	cp *pageContentOutput
+}
+
+func (p *pageOutput) enablePlaceholders() {
+	if p.cp != nil {
+		p.cp.enablePlaceholders()
+	}
+}