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

resources/page/pages.go

@@ -0,0 +1,115 @@
+// 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
+
+import (
+	"fmt"
+	"math/rand"
+
+	"github.com/gohugoio/hugo/resources/resource"
+)
+
+var (
+	_ resource.ResourcesConverter = Pages{}
+)
+
+// Pages is a slice of pages. This is the most common list type in Hugo.
+type Pages []Page
+
+func (ps Pages) String() string {
+	return fmt.Sprintf("Pages(%d)", len(ps))
+}
+
+// Used in tests.
+func (ps Pages) shuffle() {
+	for i := range ps {
+		j := rand.Intn(i + 1)
+		ps[i], ps[j] = ps[j], ps[i]
+	}
+}
+
+// ToResources wraps resource.ResourcesConverter
+func (pages Pages) ToResources() resource.Resources {
+	r := make(resource.Resources, len(pages))
+	for i, p := range pages {
+		r[i] = p
+	}
+	return r
+}
+
+// ToPages tries to convert seq into Pages.
+func ToPages(seq interface{}) (Pages, error) {
+	if seq == nil {
+		return Pages{}, nil
+	}
+
+	switch v := seq.(type) {
+	case Pages:
+		return v, nil
+	case *Pages:
+		return *(v), nil
+	case WeightedPages:
+		return v.Pages(), nil
+	case PageGroup:
+		return v.Pages, nil
+	case []interface{}:
+		pages := make(Pages, len(v))
+		success := true
+		for i, vv := range v {
+			p, ok := vv.(Page)
+			if !ok {
+				success = false
+				break
+			}
+			pages[i] = p
+		}
+		if success {
+			return pages, nil
+		}
+	}
+
+	return nil, fmt.Errorf("cannot convert type %T to Pages", seq)
+}
+
+func (p Pages) Group(key interface{}, in interface{}) (interface{}, error) {
+	pages, err := ToPages(in)
+	if err != nil {
+		return nil, err
+	}
+	return PageGroup{Key: key, Pages: pages}, nil
+}
+
+// Len returns the number of pages in the list.
+func (p Pages) Len() int {
+	return len(p)
+}
+
+func (ps Pages) removeFirstIfFound(p Page) Pages {
+	ii := -1
+	for i, pp := range ps {
+		if p.Eq(pp) {
+			ii = i
+			break
+		}
+	}
+
+	if ii != -1 {
+		ps = append(ps[:ii], ps[ii+1:]...)
+	}
+	return ps
+}
+
+// PagesFactory somehow creates some Pages.
+// We do a lot of lazy Pages initialization in Hugo, so we need a type.
+type PagesFactory func() Pages