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 #5074Fixes#5763Fixes#5758Fixes#5090Fixes#5204Fixes#4695Fixes#5607Fixes#5707Fixes#5719Fixes#3113Fixes#5706Fixes#5767Fixes#5723Fixes#5769Fixes#5770Fixes#5771Fixes#5759Fixes#5776Fixes#5777Fixes#5778
In Hugo 0.46 we made the output of what you get from resources.Get and similar static, i.e. language agnostic. This makes total sense, as it is wasteful and time-consuming to do SASS/SCSS/PostCSS processing for lots of languages when the output is lots of duplicates with different filenames.
But since we now output the result once only, this had a negative side effect for multihost setups: We publish the resource once only to the root folder (i.e. not to the language "domain folder").
This commit removes the language code from the processed image keys. This creates less duplication in the file cache, but it means that you should do a `hugo --gc` to clean up stale files.
Fixes#5058
