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

source/fileInfo.go

@@ -1,4 +1,4 @@
-// Copyright 2017-present 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"
 	"sync"
 
+	"github.com/gohugoio/hugo/common/hugio"
+
 	"github.com/spf13/afero"
 
 	"github.com/gohugoio/hugo/hugofs"
@@ -35,34 +37,46 @@ var (
 )
 
 // File represents a source file.
+// This is a temporary construct until we resolve page.Page conflicts.
+// TODO(bep) remove this construct once we have resolved page deprecations
 type File interface {
+	fileOverlap
+	FileWithoutOverlap
+}
 
-	// Filename gets the full path and filename to the file.
-	Filename() string
-
+// Temporary to solve duplicate/deprecated names in page.Page
+type fileOverlap interface {
 	// Path gets the relative path including file name and extension.
 	// The directory is relative to the content root.
 	Path() string
 
+	// Section is first directory below the content root.
+	// For page bundles in root, the Section will be empty.
+	Section() string
+
+	// Lang is the language code for this page. It will be the
+	// same as the site's language code.
+	Lang() string
+}
+
+type FileWithoutOverlap interface {
+
+	// Filename gets the full path and filename to the file.
+	Filename() string
+
 	// Dir gets the name of the directory that contains this file.
 	// The directory is relative to the content root.
 	Dir() string
 
 	// Extension gets the file extension, i.e "myblogpost.md" will return "md".
 	Extension() string
+
 	// Ext is an alias for Extension.
 	Ext() string // Hmm... Deprecate Extension
 
-	// Lang for this page, if `Multilingual` is enabled on your site.
-	Lang() string
-
 	// LogicalName is filename and extension of the file.
 	LogicalName() string
 
-	// Section is first directory below the content root.
-	// For page bundles in root, the Section will be empty.
-	Section() string
-
 	// BaseFileName is a filename without extension.
 	BaseFileName() string
 
@@ -79,14 +93,12 @@ type File interface {
 	UniqueID() string
 
 	FileInfo() os.FileInfo
-
-	String() string
 }
 
 // A ReadableFile is a File that is readable.
 type ReadableFile interface {
 	File
-	Open() (io.ReadCloser, error)
+	Open() (hugio.ReadSeekCloser, error)
 }
 
 // FileInfo describes a source file.
@@ -174,7 +186,7 @@ func (fi *FileInfo) FileInfo() os.FileInfo { return fi.fi }
 func (fi *FileInfo) String() string { return fi.BaseFileName() }
 
 // Open implements ReadableFile.
-func (fi *FileInfo) Open() (io.ReadCloser, error) {
+func (fi *FileInfo) Open() (hugio.ReadSeekCloser, error) {
 	f, err := fi.sp.SourceFs.Open(fi.Filename())
 	return f, err
 }
@@ -201,6 +213,16 @@ func (fi *FileInfo) init() {
 	})
 }
 
+// NewTestFile creates a partially filled File used in unit tests.
+// TODO(bep) improve this package
+func NewTestFile(filename string) *FileInfo {
+	base := filepath.Base(filepath.Dir(filename))
+	return &FileInfo{
+		filename:            filename,
+		translationBaseName: base,
+	}
+}
+
 // NewFileInfo returns a new FileInfo structure.
 func (sp *SourceSpec) NewFileInfo(baseDir, filename string, isLeafBundle bool, fi os.FileInfo) *FileInfo {