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

tpl/collections/apply_test.go

@@ -1,4 +1,4 @@
-// Copyright 2017 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.
@@ -29,6 +29,10 @@ func (templateFinder) Lookup(name string) (tpl.Template, bool) {
 	return nil, false
 }
 
+func (templateFinder) LookupVariant(name string, variants tpl.TemplateVariants) (tpl.Template, bool, bool) {
+	return nil, false, false
+}
+
 func (templateFinder) GetFuncs() map[string]interface{} {
 	return map[string]interface{}{
 		"print": fmt.Sprint,

tpl/collections/collections.go

@@ -1,4 +1,4 @@
-// Copyright 2018 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.
@@ -329,13 +329,17 @@ func (ns *Namespace) Group(key interface{}, items interface{}) (interface{}, err
 		return nil, errors.New("nil is not a valid key to group by")
 	}
 
+	if g, ok := items.(collections.Grouper); ok {
+		return g.Group(key, items)
+	}
+
 	in := newSliceElement(items)
 
 	if g, ok := in.(collections.Grouper); ok {
 		return g.Group(key, items)
 	}
 
-	return nil, fmt.Errorf("grouping not supported for type %T", items)
+	return nil, fmt.Errorf("grouping not supported for type %T %T", items, in)
 }
 
 // IsSet returns whether a given array, channel, slice, or map has a key

tpl/collections/collections_test.go

@@ -1,4 +1,4 @@
-// Copyright 2018 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.
@@ -311,16 +311,16 @@ func TestIn(t *testing.T) {
 	}
 }
 
-type page struct {
+type testPage struct {
 	Title string
 }
 
-func (p page) String() string {
+func (p testPage) String() string {
 	return "p-" + p.Title
 }
 
-type pagesPtr []*page
-type pagesVals []page
+type pagesPtr []*testPage
+type pagesVals []testPage
 
 func TestIntersect(t *testing.T) {
 	t.Parallel()
@@ -328,15 +328,15 @@ func TestIntersect(t *testing.T) {
 	ns := New(&deps.Deps{})
 
 	var (
-		p1 = &page{"A"}
-		p2 = &page{"B"}
-		p3 = &page{"C"}
-		p4 = &page{"D"}
+		p1 = &testPage{"A"}
+		p2 = &testPage{"B"}
+		p3 = &testPage{"C"}
+		p4 = &testPage{"D"}
 
-		p1v = page{"A"}
-		p2v = page{"B"}
-		p3v = page{"C"}
-		p4v = page{"D"}
+		p1v = testPage{"A"}
+		p2v = testPage{"B"}
+		p3v = testPage{"C"}
+		p4v = testPage{"D"}
 	)
 
 	for i, test := range []struct {
@@ -672,14 +672,14 @@ func TestUnion(t *testing.T) {
 	ns := New(&deps.Deps{})
 
 	var (
-		p1 = &page{"A"}
-		p2 = &page{"B"}
+		p1 = &testPage{"A"}
+		p2 = &testPage{"B"}
 		//		p3 = &page{"C"}
-		p4 = &page{"D"}
+		p4 = &testPage{"D"}
 
-		p1v = page{"A"}
+		p1v = testPage{"A"}
 		//p2v = page{"B"}
-		p3v = page{"C"}
+		p3v = testPage{"C"}
 		//p4v = page{"D"}
 	)