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

deps/deps.go

@@ -7,13 +7,14 @@ import (
 	"github.com/pkg/errors"
 
 	"github.com/gohugoio/hugo/cache/filecache"
-	"github.com/gohugoio/hugo/common/hugo"
 	"github.com/gohugoio/hugo/common/loggers"
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/media"
+	"github.com/gohugoio/hugo/resources/page"
+
 	"github.com/gohugoio/hugo/metrics"
 	"github.com/gohugoio/hugo/output"
 	"github.com/gohugoio/hugo/resources"
@@ -67,7 +68,7 @@ type Deps struct {
 	Language *langs.Language
 
 	// The site building.
-	Site hugo.Site
+	Site page.Site
 
 	// All the output formats available for the current site.
 	OutputFormatsConfig output.Formats
@@ -325,7 +326,7 @@ type DepsCfg struct {
 	Language *langs.Language
 
 	// The Site in use
-	Site hugo.Site
+	Site page.Site
 
 	// The configuration to use.
 	Cfg config.Provider