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/tplimpl/shortcodes.go

@@ -0,0 +1,148 @@
+// 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 tplimpl
+
+import (
+	"strings"
+
+	"github.com/gohugoio/hugo/tpl"
+)
+
+// Currently lang, outFormat, suffix
+const numTemplateVariants = 3
+
+type shortcodeVariant struct {
+
+	// The possible variants: lang, outFormat, suffix
+	// gtag
+	// gtag.html
+	// gtag.no.html
+	// gtag.no.amp.html
+	// A slice of length numTemplateVariants.
+	variants []string
+
+	info  tpl.Info
+	templ tpl.Template
+}
+
+type shortcodeTemplates struct {
+	variants []shortcodeVariant
+}
+
+func (s *shortcodeTemplates) indexOf(variants []string) int {
+L:
+	for i, v1 := range s.variants {
+		for i, v2 := range v1.variants {
+			if v2 != variants[i] {
+				continue L
+			}
+		}
+		return i
+	}
+	return -1
+}
+
+func (s *shortcodeTemplates) fromVariants(variants tpl.TemplateVariants) (shortcodeVariant, bool) {
+	return s.fromVariantsSlice([]string{
+		variants.Language,
+		strings.ToLower(variants.OutputFormat.Name),
+		variants.OutputFormat.MediaType.Suffix(),
+	})
+}
+
+// Get the most specific template given a full name, e.g  gtag.no.amp.html.
+func (s *shortcodeTemplates) fromName(name string) (shortcodeVariant, bool) {
+	return s.fromVariantsSlice(templateVariants(name))
+}
+
+func (s *shortcodeTemplates) fromVariantsSlice(variants []string) (shortcodeVariant, bool) {
+	var (
+		bestMatch       shortcodeVariant
+		bestMatchWeight int
+	)
+
+	for _, variant := range s.variants {
+		w := s.compareVariants(variants, variant.variants)
+		if bestMatchWeight == 0 || w > bestMatchWeight {
+			bestMatch = variant
+			bestMatchWeight = w
+		}
+	}
+
+	return bestMatch, true
+}
+
+// calculate a weight for two string slices of same lenght.
+// higher value means "better match".
+func (s *shortcodeTemplates) compareVariants(a, b []string) int {
+
+	weight := 0
+	for i, av := range a {
+		bv := b[i]
+		if av == bv {
+			weight++
+		} else {
+			weight--
+		}
+	}
+	return weight
+}
+
+func templateVariants(name string) []string {
+	_, variants := templateNameAndVariants(name)
+	return variants
+}
+
+func templateNameAndVariants(name string) (string, []string) {
+
+	variants := make([]string, numTemplateVariants)
+
+	parts := strings.Split(name, ".")
+
+	if len(parts) <= 1 {
+		// No variants.
+		return name, variants
+	}
+
+	name = parts[0]
+	parts = parts[1:]
+	lp := len(parts)
+	start := len(variants) - lp
+
+	for i, j := start, 0; i < len(variants); i, j = i+1, j+1 {
+		variants[i] = parts[j]
+	}
+
+	if lp > 1 && lp < len(variants) {
+		for i := lp - 1; i > 0; i-- {
+			variants[i-1] = variants[i]
+		}
+	}
+
+	if lp == 1 {
+		// Suffix only. Duplicate it into the output format field to
+		// make HTML win over AMP.
+		variants[len(variants)-2] = variants[len(variants)-1]
+	}
+
+	return name, variants
+}
+
+func isShortcode(name string) bool {
+	return strings.Contains(name, "shortcodes/")
+}
+
+func isInternal(name string) bool {
+	return strings.HasPrefix(name, "_internal/")
+}