tpl: Fix language handling in partials

[bep]

We now use the same code path for all templates re this.

Fixes #13612

[jmooring]

We now use the same code path for all templates re this.

Does this mean that partial templates are no longer language and output format agnostic?

[bep]

Does this mean that partial templates are no longer language and output format agnostic?

Well, yes and no:

• Partials gets stored the same way as other templates, but currently with a shallow path. But we use the same language/output format/media type logic as the others.
• That means that if you ask for mypartial.no.html you will get the "best matching" template based on the same rules as for shortcodes.
• But there is no "page context" when we look for partial, so e.g. mypartial will not look for "the current output format" -- but we use the values given, e.g. {{ partial "mypartial.no.html" }}.
• We also don't have a page path, so having partials in layouts/mysection/_partials/mypartial.html doesn't currently make sense, but maybe some day.

[jmooring]

Thanks for the clarification.