Allow absolute paths for template_dir when adding amp theme support

[westonruter]

If you currently add amp theme support for paired mode via:

add_theme_support( 'amp', array(
	'template_dir' => get_template_directory(),
) );

Then it will fail to find the template dir since it is expecting relative paths, it will redirect to the non-AMP version with this code:

https://github.com/Automattic/amp-wp/blob/5f635d27d64f50eeba4c0710c84d929342f967b5/includes/class-amp-theme-support.php#L675-L690

Instead, you currently have to do:

add_theme_support( 'amp', array(
	'template_dir' => './',
) );

This isn't totally intuitive. (Aside: indicating paired mode with template_dir is also not intuitive. For that, see #934.)

[westonruter]

On second thought, let's not do this. The template_dir is prepended to the currently-queried template file (e.g. single.php) and it is then passed into locate_template() which does not allow absolute paths. With #934 a theme will be able to opt-in to paired mode without having to do a pointless 'template_dir' => './' via a proposed extension to what the available_callback returns.